Topic marking particles (은/는) express what the topic of the sentence is, and subject marking particles (이/가) show what the subject of the sentence is. Object marking particles (을/를) express what is the object of the verb. “사과를” - even if you don’t say the verb, you know that 사과 is going to be the object of the verb. “ 사과가” - you know that 사과 is going to be the subject of the verb. “ 사과는” - you know that the speaker is going to say something about 사과 in comparison to other things or bring up the topic of 사과 for the first time. (source: TTMIK)