当他17岁时 可不可以说成when he was at 17 如果不行这是为什么?那应该怎么说呢?

What's your job可以说成 What job are 如果不行的话,那为什么What nationality He said he was going to Beijing 这里可不可以说He said 是主句 he was go 如果他回来 中译英如果他回来请打电话给我,这句话可不可以说if he come back ,please call me when是引导时间状语重句的,如果主句是过去时从句是过去进行时,那为什么句子是When he was young ,he when是引导时间壮语重句的,如果主句是过去时从句是过去进行时,那为什么句子是When he was young ,he when he came,i was calling,可以说成：i was calling when he came嘛? He was a good boy when he was（is) young应该选什么呢?个人认为是was. 可不可以说:When will she come?不加back 为什么? 可不可以说He is medium height ? 为什么He is medium build就可以去掉of呢?求助啊 可不可以说 He go to school by his bike?为什么? 可不可以说 he is Chinese. “他是一个美国人”可不可以说成“He is an American.