Learn Croatian: Tvoj sin nema psa. - Your son does not have a dog.

Because the dog is the object of the sentence (the thing that the son does not have), it must be in the accusative case. For masculine nouns that represent living things (animate nouns), the accusative case requires adding an -a to the end.

If we add -a to pas, why doesn't it become pasa?

The word pas has a "fleeting a". This means that when you add an ending to the word, the a inside the base word disappears. So, pas drops its middle a, then gets the new -a at the end, becoming psa.

Is nema just ne plus the verb for "to have"?

Yes! The verb imati (to have) is special in Croatian. When making it negative, the word ne merges directly with the verb. Instead of saying ne ima, it combines into the single word nema.

Why do we use Tvoj instead of Tvoja or Tvoje?

Possessive words like "your" have to match the gender of the noun they describe. Because sin (son) is a masculine noun, we use the masculine form tvoj.