Use of would in the third conditional

In the third conditional we use the past perfect in the if-clause and would in the main clause.

Don’t use would in the if-clause.

• If you would have worked harder, you would have improved.
  If you had worked harder, you would have improved.
• If I would have known you were arriving today, I would have met you.
  If I had known you were arriving today, I would have met you.