If Clauses

If Clause Type I
if + Simple Present, will-Future

Type I indicates what will happen, provided that a certain situation is given.

Examples:

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.

Examples:

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.

Examples:

If you had read the paper, you would have seen the advertisement.
If I had taken the bus, I would have been late.