1.A train leaves for Lao Cai at 5 o’clock every morning (There)

=> There is a train leaving for Lao Cai at 5 o'clock every morning.

2.The distance from HCM city to Phu Quoc Island is about 300 km(It)

=> It is 300 km from HCM city to Phu Quoc Island.

3.Is it possible to travel to Sa Pa by air? ( Can)

=> Can you travel to Sa Pa by air ?

4.Be careful or you’ll fall off your bicycle ( If)

=> If you aren't careful, you'll fall off your bicycle.