Learn Croatian: Dijete jede sir i jaje. - The child is eating cheese and an egg.

Sir is a masculine noun. In Croatian, when a masculine noun is inanimate (a thing, not a living person or animal), its accusative form is exactly the same as its dictionary form. So, sir stays sir.

Is jaje also staying the same because it is an inanimate object?

Not exactly. Jaje (egg) stays the same because it is a neuter noun. All neuter nouns have exactly the same form in the accusative case as they do in their basic dictionary form, regardless of whether they are alive or not.

Both dijete (child) and jaje (egg) are neuter nouns. How do we know which one is the subject and which is the object?

Because both the nominative (the doer) and accusative (the receiver) forms look exactly the same for neuter nouns, you have to rely on context and word order. Here, the subject (dijete) comes first, just like in English. Plus, common sense tells us the child is eating the egg!

What is the dictionary form of the verb jede?

The dictionary form (infinitive) is jesti (to eat). Here it is conjugated as jede because it matches the third-person singular subject, dijete.