ni estis = we were. ni vidis = we saw.
vi estis = you (plural) were. vi vidis = you (plural) saw.
ili estis = they were. ili vidis = they saw.


PREPOSITIONS.

36. A preposition is a word like "in," "on," placed before a noun or
pronoun to indicate some relation between this and another word. The
preposition is said to "govern" the noun or pronoun, which is called its
"complement". In English, the complement of a preposition seems to be
put in the accusative case if it is a pronoun, but to remain unchanged
in form if it is a noun. In Esperanto the preposition does not affect
the form of the word governed, which remains in the nominative case:

La arbo estas en la gxardeno, the tree is in the garden.
Bonaj pomoj estas sur gxi, good apples are on it.
Mi donis cxerizojn al li, I gave cherries to him.
La knabo estas apud mi, the boy is near me.
Sub la arbo staris cxevalo, under the tree stood a horse.


ACCUSATIVE CASE OF PERSONAL PRONOUNS.

37. For use as the object of a verb, any pronoun may be put in the
accusative case by addition of the accusative ending "-n" (23):

La viro vidis vin kaj min, the man saw you and me.
Li vidis ilin kaj nin, he saw them and us.
Mi vidis nek lin nek sxin, I saw neither him nor her.