1. This cooker, which my sister bought it yesterday, is very expensive.

→ bỏ it đi vì relative clause( mệnh đề quan hệ )which được thay thế cho cooooker nên k cần it

2 I haven't finished to decorate the Christmas tree yet.

→đổi thành decorated vì đây là hiện tại hoàn thành

3 The gas from the chemical factory is extreme harmful to our health.

→extremely

4 My son didn't worn that uniform since last year.

→hasn't worn

1. This cooker, which my sister bought it yesterday, was very expensive.

2. I haven't finished decorate the Christmas tree yet.

3. The gas from the chemical factory are extreme harmful to our healthy. 

 4. My son hasn't worn that uniform since last year.