Learn Turkish: Çöpleri ne zaman atması lazım? - When does he need to throw away the garbage?

This sentence uses the necessity structure you are learning. First, we add the short verbal noun ending -ma to the verb root at- (to throw) to make atma (throwing). Then, we add a possessive ending to show whose action is necessary. Here, the 3rd person singular possessive ending is -sı (his/her/its). So, atması lazım literally translates to "his throwing is necessary."

How do we know this means "he" and not "I" or "you" since there is no pronoun like o?

The person who needs to do the action is built right into the word atması with the -sı ending. If it meant "I need to throw," it would be atmam lazım (my throwing). For "you need to throw," it would be atman lazım (your throwing). You could put the possessive pronoun onun (his/her) at the very beginning of the sentence for emphasis, but it usually gets dropped.

Why is there an -i at the end of çöpleri?

That is the accusative case suffix, which marks a specific direct object. Because the sentence is about throwing away specific garbage ("the garbage"), we must add the -i ending to çöpler. If we were talking about throwing garbage in a general, non-specific sense, we would just say çöp.

Why is the word for garbage plural (çöpler) in Turkish when the English translation is singular?