I like a 3-story building that is a little bit far from my place. There is a table tennis club in that building. I like table tennis and the club is on the 3rd floor and it uses the whole floor so the club is big. I have to ride a bike to go there. Actually, I transferred to the new one near my place a month ago. This club is near but it is in the basement. I made a big decision to go back to the old club again. I had a hard time making a decision. Because I had to say good bye to club members. Finally, I chose the good environment rather than friendship of the club members.  

-난 종종 그들이 헷갈려요, 쌍둥이거든요.

I am confused with them. They are twins. -> I often mix them up because they're twins. => Another way: Your sentence is okay and this is another way of saying it.

-그는 나타나지 않았어요.

-He didn't show up. ✔=> Corrected: The sentence is corrected grammatically.

-학교에서 가르칠 때 하루 종일 서 있어야 합니다.

-They are standing all day long when teaching in school. -> When you teach at school you have to stand up all day. => Another way: Your sentence is okay and this is another way of saying it.