If Clause Type I
if + Simple Present, will-Future
Type I indicates what will happen, provided that a certain situation is given.
If you send your order by fax, we will deliver the goods immediately.
If you invite me for lunch, I will help you with your presentation.
If Clause Type II
if + Simple Past, would + infinitive
Type II indicates what could happen if a present situation were different.
If we had more employees, we would work more efficiently.
If we delivered poor quality, we would not be the leading company in our business.
If Clause Type III
if + Past Perfect, would + have + past participle
Type III indicates what could have happened in the past if a situation had been different then.
If you had read the paper, you would have seen the advertisement.
If I had taken the bus, I would have been late.