1. If people changed, the world _________ a better place.
a) would be
b) is
c) will be
The sentence gives you the condition clause, which is in simple past, the main clause is with would + verb.
2. People won’t use their cars if the public transportation system __________.
a) improves
b) will improve
c) improved
You need to complete the conditional clause, the main clause is in future, so the conditional is simple present.
3. If you _____________ the university, you would have found a better job.
a) left
b) hadn't lef
c) would leave
You need to complete the conditional clause, the main clause has would have + participle, so the conditional must be in past perfect.
4. We _________ swimming if it doesn’t rain.
You need to complete the main clause and the conditional is in simple present, so the result is in future.
5. If Hitler hadn’t invaded Poland, World War II ______________.
a) wouldn't have happened
b) hadn't happened
c) happened
The conditional clause is in past perfect, to complete the main clause you need would have + participle.