Here are the sentence completions:

5. I prefer to stay at home.
I would rather **stay at home.**

6. He couldn't work hard because he was very old.
He was too **old to work hard.**

7. I will not buy this bicycle because I didn't have enough money.
If I **had enough money, I would buy this bicycle.**

8. You should not tell him about this.
You had **better not tell him about this.**