Sure, here are the answers to the questions based on the correct usage of conditional sentences:

**Question 21:** If I______the rain yesterday, I______ill.
- **A.** had not caught / would have been
- **B.** had not caught / would have not been
- **C.** have not caught / were not
- **D.** have not caught / would have been

**Answer:** **A.** had not caught / would have been

**Question 22:** What______if there______a serious nuclear accident.
- **A.** will happen / was
- **B.** happens / were
- **C.** would happen / were
- **D.** would happen / had been

**Answer:** **C.** would happen / were

**Question 23:** If I hadn't stayed up so late last night, I______so tired this morning.
- **A.** don't feel
- **B.** felt
- **C.** didn't feel
- **D.** wouldn't have felt

**Answer:** **D.** wouldn't have felt

**Question 24:** I______in terrible trouble right now if you hadn't helped me.
- **A.** am
- **B.** will be
- **C.** would be
- **D.** would have been

**Answer:** **C.** would be

**Question 25:** If I had a computer, I______the assignment last night.
- **A.** finished
- **B.** have finished
- **C.** would have finished
- **D.** had finished

**Answer:** **C.** would have finished

**Question 26:** If you______harder, you'd have passed the exam.
- **A.** had studied
- **B.** have studied
- **C.** studied
- **D.** study

**Answer:** **A.** had studied

**Question 27:** If you had lots of money, what______?
- **A.** do you do
- **B.** will you do
- **C.** would you do
- **D.** have you done

**Answer:** **C.** would you do

**Question 28:** I'd______out with you if I weren't so tired.
- **A.** go
- **B.** went
- **C.** had gone
- **D.** will go

**Answer:** **A.** go

**Question 29:** I'm sure your parents would have been happier if you______harder before that important exam.
- **A.** work
- **B.** worked
- **C.** have worked
- **D.** had worked

**Answer:** **D.** had worked

**Question 30:** Tom______so careless if he'd known what would happen.
- **A.** would have been
- **B.** wouldn't have been
- **C.** would be
- **D.** wouldn't be

**Answer:** **B.** wouldn't have been

**Question 31:** If she______on time, I would have been surprised.
- **A.** arrived
- **B.** have arrived
- **C.** had arrived
- **D.** would arrive

**Answer:** **C.** had arrived

**Question 32:** We wouldn't______the bus if we'd left home earlier.
- **A.** have missed
- **B.** had missed
- **C.** will miss
- **D.** would miss

**Answer:** **A.** have missed

**Question 33:** If John had studied more, he______better marks.
- **A.** gets
- **B.** will get
- **C.** would get
- **D.** would have got

**Answer:** **D.** would have got

**Question 34:** If he had tried harder, he might______.
- **A.** succeed
- **B.** have succeeded
- **C.** be success
- **D.** will succeed

**Answer:** **B.** have succeeded

**Question 35:** If you had saved some money, you______so hard up now.
- **A.** aren't
- **B.** won't
- **C.** wouldn't be
- **D.** wouldn't have been

**Answer:** **C.** wouldn't be

**Question 36:** Why didn't you tell me? If you______me, I would have helped.
- **A.** tell
- **B.** told
- **C.** have told
- **D.** had told

**Answer:** **D.** had told

**Question 37:** If he______the money, he wouldn't be in prison now.
- **A.** hadn't stolen
- **B.** didn't steal
- **C.** had stolen
- **D.** stole

**Answer:** **A.** hadn't stolen

**Question 38:** If Tim______so fast, his car wouldn't have crashed into a tree.
- **A.** didn't drive
- **B.** hadn't driven
- **C.** haven't driven
- **D.** drives

**Answer:** **B.** hadn't driven

**Question 39:** What a bad luck! If Nam______over, he would have won the race.
- **A.** doesn't fall
- **B.** didn't fall
- **C.** hasn't fallen
- **D.** hadn't fallen

**Answer:** **D.** hadn't fallen

**Question 40:** If you______me last week, I would have been able to come.
- **A.** invited
- **B.** had invited
- **C.** would invite
- **D.** would have invited

**Answer:** **B.** had invited

**Question 41:** I'm sure your letter hasn't arrived yet. If it had come, I'm sure I______it.
- **A.** will notice
- **B.** noticed
- **C.** would have noticed
- **D.** would notice

**Answer:** **C.** would have noticed

**Question 42:** Why didn't you phone? If I had known you were coming, I______you at the airport.
- **A.** would have met
- **B.** would meet
- **C.** met
- **D.** had met

**Answer:** **A.** would have met

**Question 43:** It's a pity you missed the party. If you______, you would have met my teacher from Australia.
- **A.** came
- **B.** have come
- **C.** come
- **D.** had come

**Answer:** **D.** had come

**Question 44:** I wouldn't have resat the exam if I______harder.
- **A.** study
- **B.** studied
- **C.** had studied
- **D.** would have studied

**Answer:** **C.** had studied

**Question 45:** If Jane______, I wouldn't have been able to move the table.
- **A.** didn't help
- **B.** hadn't helped
- **C.** hasn't helped
- **D.** wouldn't help

**Answer:** **B.** hadn't helped

**Question 46:** Unless you______me about Sue's hair, I wouldn't have noticed.
- **A.** had told
- **B.** hadn't told
- **C.** told
- **D.** didn't tell

**Answer:** **A.** had told

**Question 47:** "If I______and my life depended on the solution, I would spend the first 55 minutes determining the proper question to ask, for once I know the proper question, I could solve the problem in less than five minutes." Albert Einstein
- **A.** have an hour to solve a problem
- **B.** had an hour solving a problem
- **C.** had had an hour to solve a problem
- **D.** had an hour to solve a problem

**Answer:** **D.** had an hour to solve a problem

**Question 48:** If he had spent his youth learning something, he______himself for his lack of knowledge now.
- **A.** will not blame
- **B.** would not blame
- **C.** would not have blamed
- **D.** would blame

**Answer:** **B.** would not blame

**Question 49:** If he______this homework to do, he would have gone out with his friends.
- **A.** had not
- **B.** didn't have
- **C.** had had
- **D.** hadn't had

**Answer:** **D.** hadn't had

**Question 50:** Were my grandfather alive, he______learning something new. He was really a studious.
- **A.** will be
- **B.** would be
- **C.** would have been
- **D.** will have been

**Answer:** **B.** would be