`1` I don't play the piano very well 

`2` Jack doesn't play the piano very well (Chủ ngữ Jack số ít nên dùng doesn't)

`3` You don't know the answer (Chủ ngữ You số nhiều nên dùng don't )

`4` She doesn't work very hard (Chủ ngữ She số nhiều nên dùng doesn't) 

`5` They don't do same thing everyday (Chủ ngữ They số nhiều nên dùng don't )

`->` Cấu trúc chung : S + don't / doesn't + V-inf

1(I'm not good at playing the piano

2)he's not good at playing the piano

3(you don't know the answer

4)she doesn't work hard

5(they don't do the same thing every day