1: the boy is Ben .he is playing the piano => 2:here is a from .you must fill in it => 3: the palace stands outside the town .It was built years ago => 4: there are a lot of exercises .we have to finish them now =>

1. The boy who is playing the piano is Ben

( Ben là tên người nên ta có thể dùng who)

2. Here is a form in which you must fill

( giới từ đứng trước đại từ quan hệ để tỏ sự lịch sự, hoặc bạn cũng có thể làm Here is a form which you must fill in)

3. the palace which was built years ago stands outside the town

( the palaces cũng là vật, "it" nên chia which)

4. There are a lot of exercises which we have to finish now

( bài tập => vật => which)