`1.` The dentist told him to open his mouth

`-` Form: told sb to do sth = bảo ai đó làm gì đó

`2.` I am sorry to hear that.

`-` Ta có: sorry to = xin lỗi vì điều gì

`3.` The children have played together for an hour.

`-` Thì HTHT - for an hour là DHNB, form: S + have/has + Vp.p +.

`4.` Did your school hold a song festival last month?

`-` QKĐ - last month là DHNB , form: Did + S + V-inf +.. 

`1`.The dentist told him to open his mouth

`\rightarrow` Câu mệnh lệnh : `\text{S + asked / ordered / ... + O + ( not ) to do sth}`

`2`.I am sorry to hear that

`\rightarrow` `\text{S + be + adj + to do sth}` : Như thế nào để làm gì

`3`.The children have played together for an hour

`\rightarrow` Thì HTHT : S + have / has + V3 / PII

`\rightarrow` Đặc điểm nhận dạng : for

`4`.Did your school hold a song festival last month ?

`\rightarrow` Thì QKĐ ( CNV ) : Did + S + V ?

`\rightarrow` Đặc điểm nhận dạng : last

`\text{@ TheFox}`