Learn Croatian: Vidim dječaka i čujem ga. - I see the boy and I hear him.

The word dječak (boy) is the direct object of the verb Vidim (I see), so it takes the accusative case. Since a boy is a living being, it belongs to the animate masculine group, which means we must add an -a to the end. If it were a non-living masculine thing, the word wouldn't change.

What does the word ga mean here?

ga means him. It is the short form of the pronoun on (he) in the accusative case. Because you are hearing him, he is the direct object of the verb čujem (I hear) and needs to be in this case.

Why does ga come after the verb instead of before it?

Short pronoun forms like ga are called enclitics, which are lightweight words that cannot stand on their own. They naturally want to be in the second position of their sentence or phrase. Here, the conjunction i (and) starts a new mini-sentence, so ga tucks in right after the first word of that new phrase, which is čujem.

Where is the word for I in this sentence?

In Croatian, you usually leave out subject pronouns like ja (I) because the verb endings already tell us who is doing the action. Both Vidim (I see) and čujem (I hear) end in -m, which is the standard ending for I in the present tense.