1.The food that we had in the restaurant on the corner tasted bad.
2The knife which I use to cut many things is very sharp.
3.My younger brother for whom I often buy some small gifts is a lovely boy.
改错：
1 The house in which he lives has a very beautiful garden.
2 That famous singer is the person whoI admire.
3 The girl with whom I danced just is an actress.
4 The police tried to help the girl whose mobile phone had been stolen.