The Irregular Latin Verb Sum (To Be): Conjugation, Uses, & More

The Latin verb sum (“to be”) is one of the most frequently used words in Latin, but it is also highly irregular. This post covers how to conjugate sum, how to use it, and much more.

It’s safe to say that Latin will not make much sense if you don’t understand the verb sum. It is everywhere, from the simplest Latin novellas to the most advanced Roman texts.

The forms of sum often look quite unusual, so it is important to be able to recognize them. Let’s dive right in!

How To Conjugate the Latin Verb Sum

As with any Latin verb, we will start by looking at the principal parts.

sum, esse, fuī, futūrus

The first two principal parts are different from those of other verbs you may have learned. Normally a verb’s second principal part tells you its conjugation, but esse does not have any of the standard infinitive endings.

This is because sum, esse, fuī, futūrus is an irregular verb. In other words, it does not belong to any of the four Latin conjugations.

Sum is highly irregular in the present system (the present, future, and imperfect tenses). But luckily, it is completely regular in the perfect system (the perfect, pluperfect, and future perfect tenses).

In the following sections, I will list all the forms of the verb sum. If you are a beginning student, you may want to stop reading after the present, future, and imperfect tenses and jump down to the section on how to use sum.

Present System (Indicative)

The present system includes the present, future, and imperfect tenses of the indicative mood. These are usually the first three tenses of sum that students learn.

These are also the most irregular tenses of sum, so you will have to do some memorization. But I do have some good news on that front.

Even in the present system, sum follows the expected patterns for active personal endings. That is, sum ends in m, es ends in s, and so forth.

(If you aren’t sure what this means, read about Latin verb personal endings here.)

Now let’s look at the present tense of sum and its translation.

PERSON	SINGULAR	PLURAL
1	sum I am	sumus we are
2	es you are	estis you (y’all) are
3	est he/she/it is	sunt they are

The present infinitive of sum is esse, which means “to be.”

And here is the future tense of sum and its translation:

PERSON	SINGULAR	PLURAL
1	erō I will be	erimus we will be
2	eris you will be	eritis you (y’all) will be
3	erit he/she/it will be	erunt they will be

Finally, here is the imperfect tense of sum and its translation:

PERSON	SINGULAR	PLURAL
1	eram I was	erāmus we were
2	erās you were	erātis you (y’all) were
3	erat he/she/it was	erant they were

And there you go. You have now seen the Latin verb to be in three crucial tenses. If you are a beginning student, I recommend that you now skip down to the uses of sum.

In a moment we will move on to the perfect tenses, but first we need to look at the present and imperfect subjunctive.

Present System (Subjunctive)

The present subjunctive is irregular and consists of sī– plus the active personal endings. The imperfect subjunctive, on the other hand, follows the standard rule: add the active personal endings to the present infinitive.

Here are the forms of the present and imperfect subjunctive. Subjunctives are hard to translate in isolation, but the present tense is something like I may be, you may be, etc. The imperfect tense can be translated as I might be, you might be, etc.

Person	Present	Imperfect
1 sing.	sim	essem
2 sing.	sīs	essēs
3 sing.	sit	esset
1 plur.	sīmus	essēmus
2 plur.	sītis	essētis
3 plur.	sint	essent

There is an alternative form of the imperfect subjunctive that appears in some ancient authors. It is based on the future infinitive fore (discussed below); you simply add the active personal endings to fore.

Thus we find forem for essem, forēs for essēs, and so forth.

Perfect System (Indicative)

The perfect system includes the perfect, pluperfect, and future perfect tenses of the indicative mood, as well as the perfect and pluperfect tenses of the subjunctive.

Fortunately, the perfect system of sum is completely regular. You simply add the appropriate tense and mood endings to the perfect stem.

The perfect stem, as is standard, can be found by removing ī from the third principal part. This means that the perfect stem of sum is fu-.

Here is a conjugation chart showing the perfect indicative of sum:

PERSON	SINGULAR	PLURAL
1	fuī I was / have been	fuimus we were / have been
2	fuistī you were / have been	fuistis you (y’all) were / have been
3	fuit he/she/it was / has been	fuērunt / fuēre they were / have been

And here is the pluperfect indicative of sum:

PERSON	SINGULAR	PLURAL
1	fueram I had been	fuerāmus we had been
2	fuerās you had been	fuerātis you (y’all) had been
3	fuerat he/she/it had been	fuerant they had been

And then the future perfect indicative of sum:

PERSON	SINGULAR	PLURAL
1	fuerō I will have been	fuerimus we will have been
2	fueris you will have been	fueritis you (y’all) will have been
3	fuerit he/she/it will have been	fuerint they will have been

Perfect System (Subjunctive)

Finally, let’s look at the perfect and pluperfect subjunctive. Just like with the perfect tenses of the indicative, you add the regular perfect and pluperfect subjunctive endings to the perfect stem (fu-).

It is hard to translate subjunctive forms in isolation, but the perfect means something like I may have been, you may have been, etc., while the pluperfect is something like I might have been, you might have been, etc.

Person	Perfect	Pluperfect
1 sing.	fuerim	fuissem
2 sing.	fueris	fuissēs
3 sing.	fuerit	fuisset
1 plur.	fuerimus	fuissēmus
2 plur.	fueritis	fuissētis
3 plur.	fuerint	fuissent

Infinitives, Participles, and Imperatives

The Latin verb to be is highly irregular, as we have seen, but it is also defective. This means that it does not have all of the possible verb forms. For instance, sum does not have a gerund or a supine.

The verb sum has three infinitives:

• esse = to be (present infinitive)
• fuisse = to have been (perfect infinitive)
• futūrum esse or fore = to be about to be (future infinitive)

Sum only has one surviving participle: futūrus, a, um “about to be, going to be”. This is a future participle, as you can see from -ūrus, the standard future active participial ending.

Sum has both present imperatives and future imperatives.

Person	Present	Future
2 sing.	es be!	estō be! (you shall be)
3 sing.	none	estō he/she/it shall be
2 plur.	este (y’all) be!	estōte be! (y’all shall be)
3 plur.	none	suntō they shall be

Uses of the Latin Verb Sum

In the next sections, we will look at concrete examples of the verb sum in action. You will notice that the forms of sum appear at the end, middle, and even beginning of the sentence.

In general, verbs in Latin tend to go at the end of the sentence. But sum is a bit irregular in this regard, too. The word order is highly flexible, so don’t be surprised if you spot the verb to be popping up in unlikely places.

To Be (Linking Verb)

The primary use of the verb sum is as a copula or linking verb. In this context, sum links together and equates two nouns or a noun and an adjective.

Amīca mea est fīlia agricolae. = My friend is the daughter of a farmer.

Hic puer altus erit. = This boy will be tall.

Amāre est bonum. = To love is good.

Poēta esse volō. = I want to be a poet.

We can see sum as an equals sign (=). My friend equals the daughter of a farmer; the boy will equal a tall person.

In such circumstances, the noun or adjective linked by sum (the predicate nominative) needs to be in the same case as the subject. This is typically the nominative.

(You can read more about predicate nominatives in my post on the nominative case.)

Sum can also be followed by a prepositional phrase.

Equus erat in agrō. = The horse was in the field.

Vir rogāvit cūr librī in mēnsā nōn essent. = The man asked why the books weren’t on the table.

If you want to learn more about linking verbs, then check out my post on verb types.

Existential Use of Sum

We often think of sum as simply meaning “to be,” but it can also mean “to exist.” Our English verb to be can also have this existential sense, as we shall soon see.

A classic example is René Descartes’ famous phrase:

Cogitō ergō sum. = I think therefore I am / exist.

But this usage shows up in more mundane situations, too. Forms of sum can be the equivalent of English phrases like “there is” and “there are.”

“There” is not translated into Latin. Just the verb is enough.

• est = there is
• sunt = there are
• erit / erunt = there will be
• erat = there was
• erant = there were

Take a look at the following example sentences and note the different ways that sum can be translated into English.

Sunt equī in agrō. = There are horses in the field.

Cūr est canis in sellā? = Why is there a dog on the chair?

Erant quī rēgem nōn amābant. = There were men who did not like the king. = Men existed who did not like the king.

Erit plūs pecūniae post bellum. = There will be more money after the war.

Sum as a helping verb

Sum can act as a helping verb, just like the verb to be can in English.

In English, for instance, you can say “the woman is working.” In this sentence, is isn’t acting as a linking verb or expressing existence. Instead, it is helping to convey the progressive nature of the woman’s action.

Is working is a verbal unit, which we would call the English “present progressive” tense.

Latin does not have a present progressive, but we can find the verb sum used in the passive voice. The perfect, pluperfect, and future perfect passive tenses are formed with sum and the perfect participle passive.

If you are a beginning student, then you don’t need to worry about this at all yet. If you are more advanced, then be on the lookout for sum in passive forms!

Compound Verbs Built on Sum

Sum can also form compound verbs. A prefix (usually a preposition) is added to the beginning of sum in order to change the meaning slightly.

One extremely common compound verb is possum, posse, potuī “be able, can.” It consists of the adjective potis “capable, able” plus sum. For more information, see my post on possum.

Here are some other compound verbs formed from sum:

• absum, abesse, āfuī, āfutūrus = be away, absent (used with the preposition ab + ablative)
• adsum, adesse, adfuī, adfutūrus = be near, present
• dēsum, dēesse, dēfuī, dēfutūrus = be absent, lacking
• intersum, interesse, interfuī = be between, present; differ; be of interest
• praesum, praeesse, praefuī = preside over
• prōsum, prōdesse, prōfuī = be useful, benefit (+ dative)
• supersum, superesse, superfuī = remain; abound

The good news is that all of these verbs have the exact same forms as sum, just with the prefix added. For prōsum, you insert a D between prō and the form of sum if the form begins with E (so, prōdes, prōdest, etc.).

Turris duo mīlia ab urbe abest. = The tower is two miles away from the city.

Mihi pecūnia dēerat. = Money was lacking for me (i.e. I was lacking money).

Hic liber tibi nōn prōderit. = This book will not be useful to you.

Why is the Latin verb sum so weird?

As we have seen, the conjugation of sum is very irregular. One cause of this is the fact that sum is a suppletive verb. This means that its forms go back to two entirely different roots.

The present system of sum derives from the Proto-Indo-European root *h₁es- or *es- meaning “be”. We see es or just s surviving in the present tense (sum, es, etc.), while in the future (erō, eris, etc.) and the imperfect (eram, erās, etc.) the S has changed to an R between vowels.

The perfect system and the future participle, on the other hand, come from the Proto-Indo-European root *b^huh_x– or *bheu- meaning “become” or “grow.” Eventually, the bh turns into f, and we end up with fuī, futūrus, and all the forms with fu.

The two roots *es- and *bheu- account for a lot of the confusion in the conjugation of sum.

Fun fact: the exact same thing has happened with the English verb to be. Am, are, and is come from *es-, while be, been, and being come from *bheu-. Was and were derive from a third stem, *wes- “remain, dwell.”

You may still be wondering: Why did this bizarre state of affairs occur? Historical linguistics has a lot to say on this topic, but here’s a quick answer. Basically, common words are more likely to be irregular.

Languages change over time, and irregularities in sound and form develop naturally. Over the generations speakers, just as naturally, tend to eliminate the irregularities to make words behave “normally.”

But this standardization does not happen as much with extremely common words. The reason is that speakers hear and use them constantly, so the irregular forms stick around. The verb sum is the most frequently used verb in the Latin language, so it makes sense that its peculiarities have been preserved.

Frequently Asked Questions about the Verb Sum

What conjugation is esse?

Normally when you learn a new Latin verb, you should determine its conjugation. If you know a verb’s conjugation, you know what endings it will have.

But this does not work for the verb sum, esse because it is irregular. This means that sum does not belong to any Latin verb conjugation. It has its own unique forms in the present, imperfect, and future tenses.

Is sum active or passive?

Technically, sum is neither active nor passive, because it is a copula (linking verb). But it does use active personal endings, so it has more of an active flavor.

If you need to parse verbs on a quiz or exam, I recommend that you ask your teacher what their preference is. I personally tell my students they can either omit voice in the parsing of sum OR mark it as active.

Does sum have participles?

The Latin verb sum has one surviving participle: futūrus, a, um. Futūrus, which literally means “about to be” or “going to be”, is a future active participle. Notice the familiar -ūrus ending, which signals the future active participle.

The neuter futūrum “a thing about to be” is the source of the English word “future.”

What are the principal parts of sum?

The principal parts of sum are sum, esse, fuī, futūrus. The principal parts are irregular, but they still represent the most useful forms of sum.

• Sum is the 1st person singular of the present indicative (“I am”).
• Esse is the present infinitive (“to be”).
• Fuī is the 1st person singular of the perfect indicative (“I was”).
• Futūrus is the future active participle (“about to be”).

You can read more about principal parts here.

Final Thoughts on the Latin Verb To Be

Do you feel a bit more comfortable with the verb sum and its uses now? I hope so! This irregular verb shows up everywhere in Latin, so you will have lots of opportunities to test your new knowledge.

While you’re here, you may want to take a look at the following posts on learning Latin:

Awesome Resources for Latin Learners

Best Latin Dictionaries for Students

A Guide to All My Latin Posts