"But he is smart!"
Translation:하지만 그는 똑똑해요!
Copy-pasting from the Tips & notes for those who don't have it on their phones:
There are two ways to realize "but" in Korean. First we will discuss ~지만.
~지만 is the closest the the English "but." It is attached directly to the verb stem, and indicated a contrast between the two clauses.
저는 먹지 않지만 마셔요.=I do not eat, but I do drink.
Stand alone form: 하지만 (more colloquial)/그렇지만
~는데 is a bit trickier. This conjunction is not quite like any conjunction we have in English. The basic idea is that the first clause introduces background information for the second. Often, this is a contrast, and so we translate it as "but." Sometimes, however, it is simply indicating that the two are connected, and it can be translated as "and." In its stand alone form, it may be translated as "by the way."
~는데 is attached to action verb stems and ~은데 to descriptive verbs.
저는 먹는데 안 마셔요.=> I eat, but I do not drink.
저는 한국에 갔는데, 재미있었어요=> I went to Korea and it was fun.
Stand alone form: 그런데/근데 (spoken)