Since the present is review, let’s jump into looking at the imperfect tense of sum. As with the present, the imperfect-tense forms of esse are irregular. They are:

Eram: “I was”/Eramus: “We were”

Eras: “You (s) were”/Eratis: “Y’all were/You (p) were/ You all were”

Erat: “He, She, It was”/Erant: “They were”

Notice that all these forms have as a characteristic vowel the letter a. This is the same a that shows up in the -ba- endings of other imperfect verbs. And as with other imperfect verb forms in Latin, the imperfect of the verb “to be” carries the sense of unfinished, repeated, or habitual action in the past, producing the following translations: “I was,” “I used to be,” “I kept on being”; “you were,” “you used to be,” “you kept on being”, “he, she or it was,” … Pretty obvious.

Now let’s look at the irregular forms of future:

Ero: “I will be”/Erimus: “We will be”

Eris: “You (s) will be”/Eritis: “Y’all will be/You (p) will be/ You all will be”

Erit: “He/She/It will be”/Erunt: “They will be”

Notice that these forms share a characteristic letter i which is also seen in the -bi- of other future-tense forms. And notice that they also share the same irregularities.

Now let’s look at another verb which is based upon the verb “to be,” possum. This verb ─ possum, posse ─ is a compound of sum, esse. Possum is really pot- + sum: pot- means “able”; sum means “I am.” Therefore, it literally means “I am able.” Posse is a combination of pot- + esse, meaning “to be able.”

Here is the present tense of possum:

Possum: I am able/Possumus: We are able

Potes: You are able/Potestis: Ya’ll are able

Potest: He/She/It is able/Possunt: They are able

When “t” runs into “s” in Latin, very often the t will change to an “s” and produce the geminate consonant cluster “ss.” So *pot-sum will turn into possum, *pot-sumus will turn into possumus, *pot-sunt will turn into possunt, and *pot-esse will contract down to posse.

Here is the imperfect tense of possum:

Poteram: I was able/Poteramus: We were able

Poteras: You were able/Poteratis: Y'all were able

Poterat: He/She/It was able/Poterant: They were able

Possum in the future; the expected combination of pot- + ero:

Potero: I will be able/Poterimus: We will be able

Poteris: You will be able/Poteritis: Ya’ll will be able

Poterit: He/She/It will be able/Poterunt: They will be able

Finally the complementary infinitive. In order to have a complete meaning, posse requires an infinitive, called a “complementary” infinitive because it “completes” the meaning of the main verb. Complementary infinitives serve to complete the meaning of the main verb by answering the question, "to be able to do what?" Only certain verbs take on complementary infinitives: debeo, “ought,” as in “ought to do something”; and possum, the verb introduced in this chapter, “to be able to do something.”