As an intransitive verb, esse cannot take a direct object, but serves to link the subject of a clause with a predicate noun or adjective. These are called predicate nominatives and so agree with the subject in number and case.
Future Indicative of esse
Singular
Plural
1st
érō
érimus
2nd
éris
eritis
3rd
érit
erunt
Imperfect Indicative of esse
Singular
Plural
1st
éram
érāmus
2nd
érās
erātis
3rd
érat
erant
What is possum, posse, potuī?
Possum, posse, potuī is a compound verb formed by combining the irregular adjective potis with sum. Overtime, this formed a new verb, possum, posse which means to be able or to be capable.
When using posse in the present, future, and imperfect indicative, it can be thought of as combining pot- with the conjugated form of esse. If this merger results in a ts, the t is replaced with an s, as seen below.
Present Indicative
Future Indicative
Imperfect Indicative
1st Person Singular
pós-sum
pót-erō
pót-eram
2nd Person Singular
pót-es
pót-eris
pót-erās
3rd Person Singular
pót-est
pót-erit
pót-erat
1st Person Plural
pós-sumus
pót-érimus
pót-erāmus
2nd Person Plural
pot-éstis
pót-éritis
pót-eratis
3rd Person Plural
pós-sunt
pót-erunt
pót-erant
Possum, like to be able, requires an infinitive to make sense. To be able to do what?. This is called the complementary infinitive. The complementary infinitive’s subject is the subject of the conjugated verb, so in our case here, possum.