Do you confuse with If-clause (2)-- spcial structures

We use special structures with if when we are talking about unreal situations--things that will probably not happen, situations that are untrue or imaginary, and similar ideas. In these cases, we use past tenses and would to 'distance' our language from reality.

1. if + past; would + infinitive without to
    To talk about unreal or improbable situations now or in the future, we use a past tense in the if-clause, and would+infinitive in the other part of the sentence.
     If I knew the reason, I would tell you.( Not if I know)



2. if+past perfect; would have+past participle
    This structure means past situations that did not happen.
     If he'd run a bit fast, he could have won.

3. the difference between ordinary tense-use and special tense-use 
     Take if I come and if I came for example, the difference is not necessarily a difference of time. They can both refer to the future. But the past tense suggests that a future situation is impossible, imaginary or less probable.
      If I become President, I'll…(said by a candidate in an election)
      If I became President, I'd…(said by a schoolboy) 

4. if I were
     Usually, we use were instead of was after if.

5. could and might
     could: would be able to
     might: would perhaps or would possibly