When I was watching TV,my brother did his homework.主句为什么用过去时

When I was watching TV,my brother did his homework.主句为什么用过去时 i am doing my homework w_____my brother is watching TV i did my homework instead of watching tv My brother was watching TV when my mother came back（用while改写 可以是When I was eating ,my brother was watching I am doing my homework W my brother is watching TV he was watching tv when his mother come back home用i don't kn i was watching tv while my brother was playing computer game While I was watching TV,my brother come in.（同义句） I had cut my finger when I was cooking 这里主句为什么要用过去完成时? My brother was lying in bed,watching TV.这句话中,watching TV作什么成 He was watching TV when i was washing clothes. 这句话哪错了,为什么?