To talk about unreal conditions – things that will probably not happen or situations that are untrue or imaginary – we use special tenses with if.

Present and future situations
To talk about situations that are unreal or imaginary now or in the future, we use a past tense in the if-clause and would + infinitive in the result clause. Note that a past tense is used even though the meaning is present or future.

If I knew anything more, I would tell you. (NOT If I know anything, I would tell you.) (NOT If I knew anything, I will tell you.)
If I had a car, I would be perfectly happy.

This structure can be used to make a suggestion sound more polite.

It would be nice, if you helped me with my homework.

In British English, should can be used after I and we with the same meaning as would.

If I knew his name, I should / would tell you. (British)
If we had a car, we should / would be happy.

If I were
Were is often used instead of was after if. This is common in both formal and informal speech and writing. In fact, were in more common in a formal style

If I were a little taller, I would be quite attractive. (More formal than ‘If I was a little taller,…)

Notes
In if sentences, a past tense can be used to talk about a present or future situation. It merely suggests that a situation is less probable.

For example, the sentence ‘If I got my rise, I would buy a car’ indicates less possibility of my getting a rise than the sentence ‘If I get my rise, I will buy a car’.