Based on your text, here are the corrected sentences and filled blanks for your exercise:

### Sentence Correction
7. They had to unwillingly cancel the performance because so few people in the area came to watch it.
**Correction:** "so few" is correct; no change needed.

8. In the past few months they haven’t written to themselves even once.
**Correction:** "the past few months" should remain as is.

9. They’re going to organize a very big festival to celebrate the day when the country was completely liberated.
**Correction:** "to celebrate" is correct; no change needed.

10. Those boys are interested in physics, which is the important subject in the school’s curriculum.
**Correction:** "which is" should replace "which are."

### Word Form Completion
1. He is such a (n) **unhelpful** person that no one likes him. (HELP)
2. The **celebration** is going to take place in the school yard. (CELEBRATE)
3. The scientist has made a great number of **discoveries** in physics. (DISCOVER)
4. The news was so **surprising** that they didn’t believe it. (SURPRISE)
5. **Honesty** is one of the qualities I hate the most. (HONEST)
6. His **yield** set a good example for later generations. (YIELD)
7. Among the old **reference** books, I came across a very valuable one. (REFER)
8. Vietnam is **known** for exporting agricultural produces. (KNOW)

If you have any further questions or need clarification, feel free to ask!