1. I will tell you about it when I come home from my holiday.

2. My teacher has teached English at this school for six years.( for + time -> present perfect)

3. The number of students in my class is fifty.

4. Smoke is not good for our health.

5. Why do you cry ?

Xin lỗi mấy câu khác mình không biết giải thích ntn.

I /tell/ told you about it when I come home from my holiday ( quá khứ đơn) 

My teacher /teach/ has taught English at this school for six years (hiện tại hoàn thành) 

The number of students in my class /be/ is  fifty (hiện tại đơn) 

/Smoke/Smoke  is not good for our health(hiện tại đơn) 

Why are you/cry/ crying