conditional sentences(if-clause)

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.