This is the 9700th most frequent Korean word.
고등어를
"고등어를" is the accusative form of "고등어," meaning "mackerel" in English.
In this sentence, '고등어를' signifies the object 'mackerel' being eaten. This form uses the object particle '를' to mark '고등어' as the direct object.
고등어를 먹었습니다.
I ate mackerel.
Here, '고등어를' again identifies 'mackerel' as the object, but the context of the verb '요리하고 있습니다' (am cooking) provides additional thematic information about the action related to '고등어'.
고등어를 요리하고 있습니다.
I am cooking mackerel.
In this sentence, '고등어를' indicates that 'mackerel' is the object of the verb '샀습니다' (bought), while the prefacing clause '시장에 가서' gives the contextual detail for the action.