Here are the corrections for the sentences provided:

1. Duc Giang usually doesn't **(A)** - Correct.
2. write new words **(B)** - "at" should be changed to "at."
3. Nam often drives **(C)** - "careful than" should be "more careful than."
4. his **(D)** - Correct.
5. Mr. Buong suggested **(A)** - "learning" is correct, but "learning" should be followed by "to learn" for clarity.
6. English and play **(B)** - "play" should be "playing."
7. games such as **(C)** - "Free" should be "Free."
8. at **(D)** - Correct.
9. The students **(A)** - Correct.
10. in **(B)** - "Viet Nam" should not have "in."
11. go **(C)** - Correct.
12. to school **(D)** - "on" is correct.
13. He enjoyed to **(A)** - "do" should be "doing."
14. everything **(B)** - Correct.
15. with the help of **(C)** - "of" should be "of."
16. modern equipments **(D)** - "equipments" should be "equipment."

So, the answers for the letters that need correction are: **B, C, B, A, A, B, D**.