1. It takes a long time to learn a foreign language

-> I spend a long time learning a foreign language. 

2. Give up smoking . It will make you feel better

->  The party should quit it will be better

3. It is cheaper to go by rail than by air 
-> It is more expensive to go by air than by rail.

4. You are not allowed to smoke here 
-> You must not use cigarettes at school
5. It's not very pleasant to be in hospital 
->  It's not very pleasant when in the hospital 

Đáp án:

`1.`  Learning a foreign language takes a long time.

`-` It takes + time + to do sth + O.

`=` Ving + O + takes + time.

`2.`  You would feel better if you gave up smoking.

`-` Câu điều kiện loại  `2:`

 `->` If + S + V2 / Ved + O ; S + would + V + O.

`3.`  It's more expensive to go by air than by rail.

`-` Cấu trúc `:` It's + short adj `-` er / more `-` long adj + to do sth + than + do sth.

`4.`  You musn't smoke here.

`-` S + be ( not ) + allowed + to do sth + O.

`=` S + must ( not ) + V + O.

 `->` Không được phép làm gì đó

`5.`  Being in hospital isn't very pleasant.

`-` It's ( not ) + ( adv ) + adj + to do sth + O.

`=` Ving + O + be ( not ) + ( adv ) + adj.