Here are the correct answers:

1. In many cultures, knocking on wood is **a** way to chase away bad spirits.
2. American people often watch **the** ball drop at midnight in Times Square.
3. There is at least **an** ornamental tree in my house every Tet holiday.
4. I have to go. I’m in **a** hurry.
5. Mr. Lu goes hunting in the forests twice **a** month.
6. Do you have **a** milk jug?
7. The Prime Minister will give **a** speech this afternoon.
8. There is **a** custom of doing morning exercises in my family.
9. I haven’t been to **an** open-air theatre before.
10. Here is **the** thousand pounds I borrowed from you last month.

Note: "the" is used in sentences 2 and 10 if we're referring to something specific.