1/ I suggest you should look for another job.
-> If
2/ People continue to pollute the air in the city with traffic fumes
-> The air
3/ The plane took off before we arrived at the airport
-> By the time
4/ I would prefer you not mention this matter to anyone else
-> I'd rather 
5/ My sister failed to get into a college
-> Mys sister didn't
6/ Although he wasn't experienced, he got the job
-> Despite this
7/ After 8 year, studying abroad, he return home as a doctor of law
-> After he