Third Conditional Clauses

Use Correct Tense with Third Conditional Sentences

Most English speakers have no difficulty with first and second conditional clauses, but a great many speakers get into trouble with the third conditional.
First a review.
Conditional clauses take their name from the fact that they place limits or conditions on the main clause they modify. Here are three examples of subordinate if clauses modifying main clauses:

First conditional: If you miss the bus, you will be late for school.
Second conditional: If I won the lottery, I would give my children a million dollars each.
Third conditional: If he had revealed his past before the marriage, she would not have married him.

The main clause in each sentence describes a situation that is unreal. The bus has not been missed. The lottery has not been won by the speaker. The man did not reveal his past before the marriage.
First Conditional
The situation described in the main clause is likely to happen. The if clause takes present tense and the main clause takes future tense.
Second Conditional
The situation described in the main clause could happen, but is not likely to happen. The if clause takes past tense, and the main clause takes would have + the past participle form of the verb.
Third Conditional
The if clause of the third conditional addresses a situation that did not happen in the past. The main clause speculates as to what might have happened if the action mentioned in the if clause had taken place.
In this situation, past perfect tense is used in the if clause, and present perfect in the main clause:

If you had dropped the vase, it would have shattered.

Many English speakers go to pieces with the third conditional by overdoing the woulds.
To illustrate, here are a few examples drawn from blogs, advertisements, news items, and the quoted speech of public figures.

INCORRECT: If they would have listened to the news and the weather reports, they would have been a little smarter.
CORRECT: If they had listened to the news and the weather reports, they would have been a little smarter.
INCORRECT: If they would have hired a better architect the desk area would have been closer to the plug.
CORRECT: If they had hired a better architect, the desk area would have been closer to the plug.
INCORRECT: If he would have said that, I would have thrown him out of the office.
CORRECT: If he had said that, I would have thrown him out of the office.
INCORRECT: What would have happened if they would have covered the paper with only wax and not the chitosan?
CORRECT: What would have happened if they had covered the paper with only wax and not the chitosan?
INCORRECT: Would have given 4 stars if they would have honored no spice request.
CORRECT: Would have given 4 stars if they had honored no spice request.

Remember, when the if clause refers to an action that did NOT occur in the past, use the past perfect and NOT the present perfect.
The word would should not appear in both clauses of a third conditional construction.

Source:https://www.dailywritingtips.com