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