Learn Croatian: Ona nije vidjela poruku. - She didn't see the message.

Poruka is a feminine noun, so "it" becomes the feminine accusative pronoun ju (or je). You would say Ona ju nije vidjela. Notice how the short pronoun goes right after the first word (Ona).

Why does the word for message end in -u here (poruku)?

The dictionary form is poruka. Because the message is the direct object (the thing being seen), it must be in the accusative case. For feminine nouns ending in -a, that ending changes to -u.

Why does vidjeti (to see) become vidjela here?

In the past tense, the main verb must match the gender of the subject. Since the subject is Ona (she - feminine), the verb takes the feminine singular past ending -la.

Why does the sentence use nije ("is not") instead of a word for "did not"?

The Croatian past tense is built using the helper verb "to be" alongside the main action verb. To make it negative, you use the negative forms of "to be" (like nije for he/she/it). So nije vidjela literally translates to "she is not seen", which is the standard Croatian way to say "she didn't see".