Conditional Sentences

How to Use Third Conditional Sentences
Third conditional sentences are used to explain that present circumstances would be different if something different had happened in the past. Look at the following examples:
If you had told me you needed a ride, I would have left earlier.
If I had cleaned the house, I could have gone to the movies.
These sentences express a condition that was likely enough, but did not actually happen in the past. The speaker in the first sentence was capable of leaving early, but did not. Along these same lines, the speaker in the second sentence was capable of cleaning the house, but did not. These are all conditions that were likely, but regrettably did not happen.
Note that when using the third conditional, we use the past perfect (i.e., had + past participle) in the if-clause. The modal auxiliary (would, could, should, etc.) + have + past participle in the main clause expresses the theoretical situation that could have happened.
Consider these common mistakes when applying the third conditional:
If you would have told me you needed a ride, I would have left earlier.
If you had told me you needed a ride, I would have left earlier.
Explanation: With third conditional sentences, do not use a modal auxiliary verb in the if-clause.
If I had cleaned the house, I could go to the movies.
If I had cleaned the house, I could have gone to the movies.
Explanation: The third conditional mood expresses a situation that could have only happened in the past if a certain condition had been met. That’s why we use the modal auxiliary verb + have + the past participle.
Exceptions and Special Cases When Using Conditional Sentences
As with most topics in the English language, conditional sentences often present special cases in which unique rules must be applied.
Use of the Simple Future in the If-Clause
Generally speaking, the simple future should be used only in the main clause. One exception is when the action in the if-clause will take place after the action in the main clause. For example, consider the following sentence:
If aspirin will ease my headache, I will take a couple tonight.
The action in the if-clause is the aspirin easing the headache, which will take place only after the speaker takes them later that night.
“Were to” in the If-Clause
The verb phrase were to is sometimes used in conditional sentences when the likely or unlikely result is particularly awful or unthinkable. In this case, were to is used to place emphasis on this potential outcome. Consider these sentences:
If I were to be sick, I would miss another day of work.
If she were to be late again, she would have to have a conference with the manager.
If the rent were to have been a penny more, they would not have been able to pay it.
Note that the emphatic “were to” can be used to describe hypothetical scenarios in the present, future, and past.
Punctuating Conditional Sentences
Despite the complex nature of conditional sentences, punctuating them properly is really simple!
Here’s the skinny:
Use a comma after the if-clause when the if-clause precedes the main clause.
If I’d had time, I would have cleaned the house.
If the main clause precedes the if-clause, no punctuation is necessary.
I would have cleaned the house if I’d had time.
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