For today’s post we’re going to look at the construction ~지 못하다 and ~지 않다. ~지 못하다 is used when you want to say you cannot do something, while ~지 않다 is used when you won’t do something.

Let’s take a look at some sample sentences:

오늘 숙제를 못 해요.

(오늘 = yesterday. 숙제 = homework. 를 = object marking particle. 못 해요 = cannot)

In English, this sentence would be, “[I] cannot do the homework today.”

From just this sentence, we won’t be able to tell as to why the person couldn’t do his homework. It could be that the person is physically unable to or that the person just doesn’t have enough time.

Contrast this with this sentence:

어제 숙제를 하지 않았어요.

(어제 = yesterday. 숙제 = homework. 를 = object marking particle. 하지 않았어요 = didn’t do)

In English, this sentence would be, “I didn’t do the homework yesterday”

With this sentence: 오늘 숙제를 못 해요 there’s an implication that there are some extenuating circumstances beyond the person’s control that is preventing the person from doing the homework.

However with this sentence: 어제 숙제를 하지 않았어요, the person is implying that he/she simply did not do the homework.

Knowing the difference between these two constructions is important because not using the correct construction could offend someone in a social situation.

For example, if you were to say: 생일파티에 못 가요 (생일파티 = birthday party. 에 = to. 못 가요 = cannot go)

versus 생일파티에 안 가요 (생일파티 = birthday party. 에 = to. 안 가요 = won’t go)

With this sentence:  생일파티에 못 가요 (I cannot go the birthday party), the listener could understand the sentence to mean that you would like to go, but because of some problem you can’t, while 생일파티에 안 가요 (I won’t go to the birthday party) could be taken as, you don’t wish to go, and so you won’t go to the party. This is why the first sentence may be a politer way to reject someone than the second sentence.