Learn Croatian: Trebam komad papira. - I need a piece of paper.

papira is in the genitive case. We use the genitive here to translate the "of" in "a piece of paper". Because papir (paper) is a masculine noun, it takes the -a ending in the genitive singular, which is the main focus of this lesson.

Why didn't komad change its ending, since it is the object of the verb?

komad (piece) is the direct object of Trebam (I need), so it is in the accusative case. However, for masculine nouns that are non-living things, the accusative form looks exactly the same as the basic dictionary form. That is why it does not change.

Where is the word for "I" in this sentence?

In Croatian, you usually do not need to use subject pronouns like ja (I). The verb ending does the job. The -am at the end of Trebam already means "I need". You would only add ja if you wanted to emphasize that you need it, not someone else.

Will I always use this -a ending after words like "a piece of"?

Yes, for masculine and neuter nouns! Words that express quantity or measurement are always followed by the genitive case. For example, if you want to say "a piece of cheese", you take the masculine noun sir (cheese) and add -a to make it komad sira.