Questions & Answers about ראינו את הספינה אתמול.
ראינו means we saw.
It is a past-tense verb form from לראות (to see). In this sentence, the verb already includes the subject we, so one Hebrew word covers what English says with two words: we saw.
Because Hebrew often puts the subject information inside the verb itself.
So ראינו already tells you we saw, and Hebrew does not need אנחנו (we) unless the speaker wants extra emphasis or clarity.
You could say אנחנו ראינו את הספינה אתמול, but it is not necessary.
Here את is the direct object marker. It comes before a definite direct object.
So in ראינו את הספינה, the thing that was seen is the ship, and because the ship is definite, Hebrew uses את before it.
It usually does not get translated into English. It is a grammatical marker, not a word with a meaning like to or with.