1. They last wrote to each other five years ago 2. He started to learn Ẹnglish when he was in grade 6 3. I haven't visited the museum for three months

=>

1. They haven't written to each other for five years.

2. He has learned English since he was in grade 6.

3. The last time I visited the museum was three months ago.

1. They haven't written to each other for five years.

-S + have/ has + V3 + O

-  for five years- Thì HTHT

2. He has learned English since he was in grade 6.

- sine -Thì HTHT: S + have/ has + V3 + O

3. The last time I visited the museum was three months ago

- ago - Thì QKĐ - The last time + S + Verb (Simple past tense/ present perfect tense)