There are several ways to say “but” and “however” in Korean.

One way is to use 그렇지만 :

호동은 한국인입니다. 그렇지만 한국어를 못 합니다.

Hodong is a Korean person. However/but he can’t speak Korean.

호동 = Hodong. 은 = topic marking particle. 한국 = Korea. 인 = person. 입니다 = is.

그렇지만 = but/however. 한국 = Korean. 어 = language. 를 = object marking particle. 못 합니다 = can’t.

One thing to note is that 그렇지만 is pronounced as 그렇치만 by Korean speakers.

하지만 can also mean “but” or “however” :

유지니는 밥을 많이 먹어요. 하지만 살이 찌지 않아요.

Yujin eats a lot (literally eats a lot of rice) but/however she doesn’t gain weight.

유지니 = Yujin. 는 = topic marking particle. 밥 = rice. 을 = object marking particle. 많이 = a lot. 먹어요 = eat.

하지만 = but/however. 살 = weight (literally means “flesh”). 이 = subject marking particle. 찌지 않아요 = doesn’t gain/doesn’t get fat.

하지만 is used in more informal situations.

그러나 can also be used to mean “but/however” :

어제 열심히 공부했습니다. 그러나 아직도 시험에 떨어졌습니다.

I studied hard yesterday, but I still failed the test.

어제 = yesterday.  열심히 = hard. 공부했습니다 = studied.

그러나 = but/however. 아직도 = still. 시험 = test. 에 = locative particle. 떨어졌습니다 = failed.

그러나 is used often in written speech and is more formal than 하지만.