Question

Помогите пожалуйста: 1) Yan (just /write) the letter to his cousin.2)He (buy) these stamps last month​

Answer 1

1) Yan has just written the letter to his cousin. (есть слово-маркер just, используем Present Perfect)

2) He bought these stamps last month​. (действие произошло в прошлом, время действия указано, это Past Simple)

Answer 2

1. Yan just wrote the letter to his cousin.
2. He bought these stamps last month.