"Teaching a friend is hard."
Translation:친구를 가르쳐주는 것은 어려워요.
가르치는 것 is just the basic way to say "thing that teaches" or "teaching"
가르쳐주는 것 gives it the feeling of the thing that is being taught is being taught as a favor or for someone/something
주다, which is at the end of the 가르치다 stem, means "to give."
So, although this wouldn't be an accurate translation, you could think of anything with 주다 at the end to be like this:
친구를 가르쳐주는 것은 어려워요.
The teaching of a friend, which is being done as a favor, is difficult.
Here's another example of the 주다 verb attached to another verb:
그 원피스 참 예뻐서 제 친구 샀어요.
Because that dress is very pretty, my friend bought it.
그 원피스 참 예뻐서 제 친구 사줬어요.
Because that dress is very pretty, my friend bought it for me.
Wanting to unlock a lesson to gain time and then failing due to the app randomly requiring subjects like "na n'n" where they could be skipped and then rejecting "him d'reoyo" in a sentence where it's arguably making more sense then "eoryeowoyo" is annoying! The Korean course might have the funniest example sentences (and the nicest voice) but tranlation flexibility is awefully insufficient!