1. While we were on holiday, we were spending most of our time sightseeing. 
Sửa: While we were on holiday, we spent most of our time sightseeing. 
2.  Paul listened to his iPod, so he didn't hear the doorbell ring.
Sửa: Paul was listening to his iPod, so he didn't hear the doorbell ring.
3.  He is a most intelligent student in our class.
Sửa: He is the most intelligent student in our class.
4.  Your new car is more cheaper than John's.
Sửa: Your new car is cheaper than John's. (bỏ more)
5.  He left college when he is 17.
Sửa:  He left college when he was 17.