Learn Croatian: Nije se javio jer se nije sjetio. - He didn't answer because he didn't remember.

You use the negative forms of the helper verb "to be" (nisam, nisi, nije, etc.) plus the past action word. Here, we use nije (he/she/it didn't) with the masculine past words javio and sjetio. We do not use the word ne separately in the past tense.

There is no word for "he" here. How do we know who didn't answer?

You can tell it is a "he" because of the -o ending on the past action words (javio and sjetio). This ending is strictly for masculine singular subjects. If the sentence was about a woman, it would be Nije se javila jer se nije sjetila.

Why does se come after nije in the first half, but before nije in the second half?

se is a type of short word (a clitic) that generally wants to be in the second position of its phrase. In the first part, Nije is the first word, so se comes second. In the second part, the word jer (because) starts a new phrase, so se jumps right after it into the second spot, pushing nije into the third spot.

What exactly does the verb javiti se mean?