-고 나서

V + 고 나서 (고 + 나다 + -아서) This indicates that the act of the preceding clause is over and the act of the following clause takes place (and then).

-고 나서 is similar to -고(and then), but it puts slightly more emphasis on the completion of the preceding action. (It’s not exactly a connective ending, but it functions in a similar way.)

숙제하고 나서 놀았어요.

I played after doing my homework.

(Compared to "숙제하고 놀았어요", this emphasizes that I played after completely finishing the homework.)

손 씻고 나서 밥을 먹으세요.

Wash your hands before eating.

밥을 먹고 나서 이를 닦았어요.

I brushed my teeth after eating.

• -고 나면: if you finish~, and then…

숙제를 하고 나면 더 잘 이해될 거예요

If you do the homework then you will understand it better.

• -고 나니까: I finished~, and then I realized/felt…

숙제를 하고 나니까 더 잘 이해되었어요.

After doing the homework, I understood it better.

• Tense: always base form

숙제 했고 나서 놀았어요. X → 하고 나서 O

숙제 하겠고 나서 놀 거예요. X → 하고 나서 O