Talking about unreal situations
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,…)
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’.