1What were you doing when the plane rang?
2.They didn't leave for home until they had finished the work.
3.Where are the Ming Tombs
4.When these students bought many things,they began thein trip.
5.I couldn't help playing one last little joke on you.
We cannot help being impressed by their zeal.我们不由得不被他们的热情感动.
He knows nothing about it,so he can’t help to do any of your work.
can’t help to do (不能帮助做某事) can’t help doing sth = 禁不住做某事
could't help doing sth.用在过去时,can't help doing sth 用在一般现在时.