Third Conditional

 And here comes the third conditional. What's important to remember about this conditional?

a) The verb in the if-clause is in the past perfect tense

b) The verb in the main clause is in the perfect conditional.

c) The time is past and the condition cannot be fulfilled because the action in the if-clause didn't happen.

• If I had known that you were coming, I would have met you at the railway station.
  But I didn't know that you were coming so I didn't come.
  
• If he had tried to leave the country, he would have been stopped at the frontier.
  But he didn't try.

Possible variations of the basic form:

1. could or might may be used instead of would:

• If the rescue crew had found him earlier, they could have saved his life. (ability)
• If the rescue crew had found him earlier, they might have saved his life. (possibility)
• If we had the necessary documents, we could have left at once. (ability or permission)

2. The continuous form of the perfect conditional may be used:

• If I'd had any money I'd have been watching the film with my girlfriend that evening.

3. We can use the past perfect continuous in the if-clause:

•   I wasn't wearing a seat belt. If I had been wearing one, I wouldn't have been seriously injured.

4. A combination of types 2 and 3 is possible:

• The airplane I intended to catch crashed.  If I had caught that airplane,  would have been killed or I would be dead now(type 3)

• If he had worked harder at school, he would be working in a comfortable office now; he wouldn't be sweeping the streets. (But I didn't work hard at school and now he is sweeping the streets.)

5. "had" can be placed first and the "if" omitted:

For example, instead of saying:

• If you had obeyed orders this disaster would not have happened.

we can say:

• Had you told me about your problems,  this disaster would not have happened.