六、用括号内所给动词的正确形式填空(共5小题;每小题l分，满分5分)

　　49.I didn’t know how ______________(teach) children when I started to work in a primary school.

　　50. --Why didn't you come to open the door for me?

　　---I (wash) dishes in the kitchen and didn't hear the doorbell.

　　51. My computer fan doesn't work. Where can I have it_________ (repai r)?

　　52. I don't think he stole the painting. When he__________ (ask) what he was doing at 7 yesterday evening, he was very calm.

　　53. --Is there any mistake in your homework, Jim?

　　--I'm not sure. I (not check) it carefully.