Here’s the reordered version of the sentences:

1. What do you often decorate your Christmas tree with?
2. You have to study harder, or you may fail the exam.
3. The Vietnamese prepare offerings during Tet holiday to worship their ancestors.
4. If teenagers play too much online games, they will lose their concentration.
5. Ann is very tired; however, her mother helps with the housework.
6. If teenagers put private information on social media networks, it could be dangerous for bad purposes because people may use the information.