• If + past perfect, subject + would have / would not have + past participle.
• If I had walked home, I would have arrived by now.
• If I had waited for the bus, I would not have arrived home yet.
• Subject + would have / would not have + past participle + if + past perfect.
• I would have arrived home if I had walked.
• I would not have arrived home yet if I had waited for the bus.

Replacing Would

Above, the example used would but you can replace would with:

• May have or may not have.
• She may have caused the crash if she had driven to fast.
• Might have or might not have.
• If I had not gone out that night I might not have met my husband.
• Could have or could not have.
• If I could have helped, they would have completed everything on time.

Replacing If

Replace if with I wish or if only.

• I wish
• I wish I would have helped so they would have completed everything on time.
• If only
• If only I could have helped, then they would have completed everything on time.

When the If clause Is Not Said.

Occasionally the if clause is implied.

• I would have helped, if you have asked me. I would have helped.

Talking About Situations That Never Happened.

• If he had gone to university, he might have received a higher wage.
• What would you have done if you hadn't taken this job?
• If I had run faster, I would have finished the race by now.

Expressing regret.

• I wish I had worked harder so I could have a better job now.
• If I hadn't eaten so much, I wouldn't have a stomach ache.
• I wish we have saved more money, we might have retired earlier.

Expressing Relief.

• I would not have fallen if I had stayed at home.
• If we had driven along the other road, we would have been stuck in a traffic jam.
• If I had worn that dress, I would have looked awful.

To criticise.

• This country would have been a nice place if the people had kept it clean.
• If she hadn't driven so fast, she might not have had her accident.
• If I had known the movie would be so bad, I wouldn't have come.