The Type 3 Conditional - Alternate forms

Here we use a past perfect in the if-clause and would have + past participle in the result clause.

• If I had won that award, my life would have changed. 
• If she had studied science, she would have found a better job. 

The Type 3 Conditional is used to talk about past situations that did not happen.

• If you had invited them they would have come. (You did not invite them and therefore they did not come.) 
• If you had worked harder you would have passed the exam. (You did not work hard and therefore you did not pass.) 

Modals in result clauses

Could have, might have, should have etc., can be used in place of would have.

• If he had known, he could have helped you.
• You might have finished the assignment on time,if you had planned more carefully. 
• If you hadn't prepared, you should have told the professor. 

But for

But for replaces if not and is followed by a noun. It is usually used in formal speech.

• But for our savings, we wouldn't have been able to make the payments.