Learn Croatian: Stvarno te nismo vidjeli jučer. - We really didn't see you yesterday.

te is a short, unstressed pronoun (a clitic) meaning "you" in the accusative case. In Croatian, clitics strongly prefer to be the second element in a sentence. Since Stvarno is the first word, te tucks in right behind it.

Normally the helper verb for "we" is the clitic smo. Why doesn't nismo act like a clitic here?

While the positive helper verb smo is a clitic and must go in the second position, its negative form nismo is a fully stressed word. Because it is not a clitic, it does not have to follow the strict "second position" rule, leaving te as the only clitic in this sentence.

Why does the sentence use the accusative case for "you"?

The verb vidjeti (to see) requires the accusative case because "you" is the direct object of the sentence—the person receiving the action of being seen.

How do we know who "we" are based on the word vidjeli?

The -i ending on past tense verbs is used for a group of males or a mixed group of males and females. If a group of only females were speaking, they would use the feminine plural ending and say vidjele.