If Clause Type I
if + Simple Present, will-Future
Type I indicates what will happen, provided that a certain situation is given.
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 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 I had taken the bus, I would have been late.