Here are the completions for the sentences with "should" or "shouldn't":

1. You **should** buy peach flowers at Tet.
2. You **should** clean and decorate your house before Tet.
3. You **shouldn't** break the things at Tet.
4. You **should** wish all the people you know "Happy New Year."
5. You **should** help your mother cook special food.
6. You **shouldn't** quarrel with each other.
7. You **should** go to a pagoda to pray for good health and good luck.
8. You **shouldn't** play cards for money.
9. You **shouldn't** stay up late.
10. You **shouldn't** watch too much TV.