I am watching TV when suddenly my mother comes in.
我正在看电视,妈妈突然进来了。
I am going to watch TV when suddenly my mother comes in.
我正要去看电视,妈妈突然进来了。
I will watch TV when suddenly my mother comes in.
我即将去看电视,妈妈突然进来了。
I was watching TV when suddenly my mother came in.
我刚刚在看电视,这时妈妈突然进来了。
be doing…when…
be about to do…when…
be on the point to do…when…
have done…when…
对应:
am going to=be~about to do
on the point of~will
have done~was watching