Here are the corrected sentences with the mistakes underlined:

6. We haven’t visited our family **since** 2 years.
**Correction:** We haven’t visited our family **for** 2 years.

7. Mr. Tom **teaching** them when they were freshmen in the university.
**Correction:** Mr. Tom **was teaching** them when they were freshmen in the university.

8. She has never **ate** sushi before, so she's excited to try it.
**Correction:** She has never **eaten** sushi before, so she's excited to try it.