"The man rides on a pig."
Translation:남자가 돼지를 탑니다.
Because 을/를 is an object marker. Whatever it is attached to is not the main focus of the sentence or is not performing any action. The subject marker 이 (for nouns ending in consonants) and 가 (for nouns ending in vowels) is the thing/person doing the action. Sentence structure is korean is subject/object/verb. It took me a long time to identify these parts of the sentence and to learn their order. Keep practicing. Hope this helps.