Learn Croatian: Trebam deset grama šećera za kolač. - I need ten grams of sugar for the cake.

This is the genitive plural! In Croatian, numbers 5 and up (like deset) are always followed by the noun in the genitive plural. For most regular masculine nouns like gram, you form the genitive plural by adding the ending -a.

šećera also ends in "-a". Is it also in the genitive plural?

Actually, no. šećera is in the genitive singular. We use the genitive case here to mean "of sugar". Since sugar is an uncountable substance when baking, it stays singular. It just happens that both the genitive singular and genitive plural for masculine nouns often share the -a ending.

There is no word for "of" in this sentence. How do we know it means "grams of sugar"?

The genitive case does that job for us! Putting a noun in the genitive often translates directly to "of that thing" in English. So putting šećer into the genitive form šećera naturally means "of sugar" without needing an extra preposition.

Why doesn't the word for cake (kolač) change its ending after the preposition za?

The preposition za (when it means "for") requires the following noun to be in the accusative case. Because kolač is a masculine inanimate noun (a non-living object), its accusative form is exactly the same as its basic dictionary form.