用所给词的适当形式填空。
1. —You look ______ (worry). What's happened?
—Oh, I can't find my English book.
2. Two ______ (visit) have just arrived, they are ______ (America).
3. —How long ______ you ______ (study) English?
—For two years.
4. They (not hear) of that thing yet.
5. —When he (return) it?
—Half an hour ago.
6. ______ (thank) to your help, I can find my dog.
7. —______ he ______ (be) to Beijing before?
—Yes, he has.
8. I ______ never ______ (see) such a wonderful film before.
9. I happened _______ (meet) my old friend in the street yesterday.