What you eat, drink and watch

This is the third particle that changes with the final letter, after 은/는 on day 3 and 이/가 on day 4, and the logic is the same each time. After a consonant, the form that starts with a vowel (을) lets the consonant link across into it; after a vowel, the form that starts with a consonant (를) stops two vowels running into each other. So you never memorise which nouns take which: you listen to how the noun ends. Whatever form it takes, 을/를 marks the thing the action is done to.

Korean questions keep the shape of the answer. English moves 'what' to the front and adds 'are you'; Korean puts 뭐 where the object would be and changes nothing else, so 뭐 먹어요? is answered by 김밥을 먹어요 with one word swapped. The polite present covers both 'I eat' and 'I am eating', so 뭐 해요? can be either question and context decides. Notice also that 뭐 usually goes without its particle; the full 뭐를 is rare in speech.

In English, word order is the grammar: 'the dog bites the man' and 'the man bites the dog' differ only in order. In Korean the particles do that job, so the nouns can move around without the meaning breaking, as long as the verb stays at the end. The flip side is that when the roles are obvious, the particles become optional and speakers drop them. Nobody drinks a person, so 커피 마셔요 needs no 를. Use the particles while you are learning; dropping them later is easy, putting them back is not.