Learn Croatian: Zatim je završila projekt. - Next, she finished the project.

je is an auxiliary (helper) verb used to make the past tense. It is also an enclitic—a small word without its own stress. In Croatian, enclitics strictly follow a rule called Wackernagel's law, which means they must be the second element in a sentence. Since Zatim takes the first spot, je must immediately follow it, even if it separates the helper verb from the main verb.

How do we know it means "she finished" when there's no word for "she" in the sentence?

In Croatian, you usually drop subject pronouns like ona (she). You can tell who did the action by looking at the l-participle (the main past tense verb). Because završila ends in -la, it tells us the subject is feminine and singular. If it were "he finished", the verb would be završio.

If projekt is the direct object of the sentence, why didn't its ending change?

You're right that direct objects take the accusative case, which often means changing the ending of the word. However, projekt is a masculine inanimate noun (a non-living thing). For masculine inanimate nouns, the accusative case form is exactly the same as the basic dictionary (nominative) form.

What if I wanted to include the word ona (she) for emphasis? Where would it go in the sentence?