1. The copula sum, esse, fuī, futūrus
To be
The copula (sum, esse, fuī, futūrus);
-us,-a,-um (1st and 2nd declension) adjectives
XVIa. Sō far, we expect a sentence to show a subject (either present or implied) and a verb, and if the verb is transitive, a direct object.
e.g. Pirata pisces vendit.
Or, there is just a subject and verb if the verb is
intransitive: hospes (per forum) currit. Or
passive: rosa (ab ancilla) carpta est.
There is one verb that acts sō differently that it could be considered nō verb at all.
The word to be, sum, esse, fuī, futūrus in Latin, does not take a direct object but rather operates as an expression of equivalence.
The irregular forms are
Indicative sing. plural
Present est sunt
Imperfect erat erant
Future erit erunt
Subjunctive(we have not learned any use for the present subjunctive at this point)
Present sit sint
Imperfect esset essent
Lucius erat asinus. Lucius was a donkey
Lucius in Thessaliā fuerat hospes. Lucius had been a guest in Thessaly.
Photis erat ancilla. Photis was a slave girl
Dominae sunt veneficae. The mistresses are witches

2. So far, we expect a sentence to show a subject (either present or implied) and a verb, and if the verb is transitive, a direct object.
e.g. Pirata pisces vendit.
Or, there is just a subject and verb if the verb is
intransitive: hostis (per forum) currit. Or
passive: rosa (ab ancilla) carpta est.

3. to be: sum, esse, fuī, futūrus
There is one verb that acts so differently that it could be considered no verb at all.
To be: sum, esse, fuī, futūrus does not take a direct object but rather operates as an expression of equivalence.

4. to be: sum, esse, fuī, futūrus
By equivalence is meant that the verb can be described as an equal sign, and should be marked thus. Equal sign because the things on either side of the verb describe (or simply are) the same thing.
Lucius erat asinus.
Lucius was a donkey
Lucius in Thessaliā fuerat hospes. Lucius had been a guest in Thessaly.
Photis est ancilla. Photis is a slave girl

5. By equivalence is meant that the two terms refer to the same thing
Lucius erat asinus.
Lucius is the same as the donkey
Lucius in Thessaliā fuerat hospes.
Lucius is the same as the guest in Thessaly.
Photis est ancilla.
Photis is the same as the slave girl

6. Mark-up with an = Thus there is no accusative, but two nominatives:
S = S
Lucius erat asinus.
Some refer to the second S as P (for predicate), but that is not necessary.
Since this is no ordinary verb, we will use the =

7. Forms of sum, esse, fui, futurus
The forms (irregular) are
Indicative sing. plural
Present est sunt
Imperfect erat erant
Future erit erunt
Subjunctive
Present sit sint
Imperfect esset essent
(we have not yet learned any use for the present subjunctive)