Hello.
1.She didn't understand that exercise because it was very difficult.
2.They built a house five years ago because they won the lottery.
3.Jack knew how to do this exercise because it was easy.
4.My sisters were happy when they met me.
5.They can't help her because she won't pay them.
6.Elen drank all the milk last night because she was thirsty.
Pozdrawiam
Dobranoc :)