用所给动词的适当形式填空。
1. We were looking forward to____ (visit) Shanghai World Expo .
2. The law of gravity____ (discover) by Isaac Newton about 350 years ago.
3. You must try your best to stop him____ (make) the same mistake again.
4. Jack said he____ (return) the two books to the library in a week.
5. He prefers____ (take) part in the public activities rather than stay at home alone.
