You never know how much difficulty we had _________________ the problem. Luckily, we try to solve it with Mrs Wu’s help.
____________ they are brothers, they look like each other.
We will spend our summer holiday in Guilin, but we haven’t decided______________.
— When _________ you________ your teeth, Billy? —Just now.
When I was walking in the street, I saw a plane___________ over my head.
We ___________ when the teacher came in.