指出下列句子的错误并改正
问题描述：
指出下列句子的错误并改正
1.He spends two hours to do the homework every day.
2.My sister was reading when my brother was playing.
3.I was playing the violin while there was a knock on the door.
4.One of the most interesting thing in summer is swimming.
答
1.the去掉
2.when变成while
3.while变成when
4.thing 加s