3 My's sister is shorter than her 

`->` S + tobe + ADJ + er + than ...

4 Hoa is the most intelligent student in my class

`->` S + tobe + the most + ADJ......

5 This song is so long that we can't learn it by heart 

`->` S + tobe + so + adj + that + S +V

6 He is interested in watching TV 

`->` S + tobe + interested in + Ving 

7 I spend 20 minutes  going to school everyday.

`->` S + spend + Ving

8 Going out at night it's dangerous

--Ngoc--

`3`. My's sister is not as tall as her.

`-` So sánh bằng: S+ be+ ( not)+ as+ adj+ as_ O

`4`. Hoa is the most intelligent student in my class.

`-` So sánh nhất ( adj dài vần): S+ be+ the+ most+ adj

`5`. This song is so long that we can't learn it by heart.

`-` Cấu trúc: Quá rằng: S+ be+ so+ adj+ that+ SVO

`6`. He is interested in/ keen on watching cartoon.

`-` be+ interested+ in+ Ving

`=` be+ keen+ on+ Ving

`=` be+ fond+ of+ Ving: thích làm gì

hoặc: Watching cartoon is his hobby.

`-` Cấu trúc: Ving+ be+ one's+ hobby

`7`. I spend `20` minutes going to school every day.

`-` spend+ time+ Ving: để dành TG làm gì

`8`. Going out at night is dangerous.

`-` Cấu trúc: It's+ adj+ ( for sb)+ to V

`≈` Ving+ be+ adj