The 4 Types of Conditional Sentences

Zero-Type Conditional

The form of the zero conditional causes no problems since the present tenses are used in both clauses, 

Zero-type conditionals
If clause	Main or conditional clause
If + Present tense	Present tense
If you heat water	it boils.

The zero conditional is normally used to talk about facts and to express general truths.

First-Type Conditional

The basic form for this type of conditional sentence can be seen in the chart below. As before, the order of the clauses can be changed with no change in meaning.

First-type conditionals
If clause	Main or conditional clause
If + Present tense	Future tense
If they don't arrive soon If they are late	we'll leave without them. I'm going to be angry.

You will note that on the if side of the sentence any present tense can be used, while in the main clause the speaker is free to choose any future that helps to express any additional meaning that the speaker wants to express.

If he's sleeping, he will not wake up until morning. (The present continuos in the first part of the sentence expresses the present temporary nature of the situation and the will in second part is making prediction about the future).

Alan is going to post me the recipe, if he finds it. (In the first clause I am expressing Alan's intention so going to is the best future to use, while the second clause contains  a simple present tense).

If he's staying at the party, I'm leaving. (In the first clause I am thinking about the possible current state of affairs, so I choose the present continuos, while in the second  I am reffering to the future plan that I have in mind should he decide to stay, so again I choose the present continuos).

If you have finished the essay, leave it on my desk. (By using the present perfent tense in the if clause I am stressing the completed nature of the action, while in the second clause I have used an imperative, which has a future meaning).

Second-Type Conditional

This type is often called the hypothetical or 'unreal' future conditional since it is usually used to speculate about either very unlikely future situations or present and future impossibilities.

Second-type conditionals
If clause	Main or conditional clause
If + Past tense	would + verb
If I had time If I had wings	I would drop you off at school. I would fly.

  

Third-Type Conditional

This type refers to hypotetical situations in the past. In this case we use the past perfect tenses in the if clause and would + have in the main clause, 

Third-type conditionals
If clause	Main or conditional clause
If + Past Perfect tense	would have + past participle
If I had known about his condition If we had known about the storm	I would have phoned for you earlier. we wouldn't have started our journey.

The main uses of the third conditional are for speculating about the past, expressing regrets, excusing our own actions and criticising others. 

https://www.tesol-direct.com/guide-to-english-grammar/conditional-sentences

 

Conditional Sentence

Exercise 21: Conditional Sentence (Page 97)       Exercise 22: Used To (Page 99)

1. were understood                                                  1. eating
2. would not have been                                           2. eat
3. can give                                                               3. swim
4. would have told                                                   4. like
5. would have been                                                  5. speaking
6. had                                                                       6. studying
7. could stop                                                            7. dance
8. needed help                                                          8. sleeping
9. would have found                                                9. eating
10. enjoyed                                                              10. eating
11. paint              
12. were                                                                    Exercise 23: Would Rather (101)
13. write
14. could have                                                           1. stay
15. were spending                                                     2. have stayed
16. will accept                                                           3. work
17. buy                                                                       4. study
18. had decided                                                          5. not study
19. would have written                                              6. have
20. leaks                                                                     7. have stood
21. had studied                                                           8. not cook
22. has hear                                                                9. had not arrived
23. see                                                                        10. have slept
24. gets
25. have turn                                                              Exerc.. 24: Must/ Should + Perfective (Pg. 105)
26, were
27. would have called                                                1. should have had
28. would have talked                                                2. must have been
29. explained                                                              3. must have damaged
30. spoked                                                                  4. must not have parked
                                                                                   5. must have studied
                                                                                   6. should have studied
                                                                                   7. must have been
                                                                                   8. should have deposited
                                                                                   9. must have forgotten
                                                                                   10. must not have studied

                                                                                   Exercise 25: Modals + Perfective (Page 105)

                                                                                   1. I would
                                                                                   2. would have gone
                                                                                   3. may have had
                                                                                   4. should have done
                                                                                   5. must have forgotten
                                                                                   6. may have slept
                                                                                   7. might have had
                                                                                   8. could have lost
                                                                                   9. should not have driven
                                                                                   10. may have run