These sentences relate to unlikely or unreal situations.

We simply imagine or sympathise ourselves if the situations would be better or otherwise than what they are.

• If I were you, I wouldn’t buy this coat.
• If I were you, I wouldn’t tolerate his arrogance.
  (This is a present situation and I’m not in your place)

In the above two sentences, you MUST use ‘If I were you’ invariably. (NOT ‘If I was you’) probably because, on account of frequent repetition, it has become a set expression.

BUT,

• If he was/were a bird, he would fly.
  (Here ‘was’ or ‘were’ is possible)
• If he was/were a millionaire, he would buy an aeroplane.
• If you asked (not ‘ask’) for details, he could give you.
  (i.e., you are not asking for details and so he is not giving details)
• If he came now, we could start the meeting.
  (Alas! he hasn’t come yet)
• If I had (not have) enough money, I could help you.
  (i.e., I haven’t enough money now)
• If your father was/were here, he wouldn’t tolerate your impudence.
• If it weren’t/wasn’t raining now, we could start our journey.

NOTE – In all the above sentences, there are two clauses viz (a) ‘If Clause’ or Conditional Clause and (b) ‘Main Clause’. In the ‘If Clause’ we use simple past form of the verb ( such as was, were, asked, had etc) and in the Main Clause, the future-in-the-past (would, could, should, might etc.) should be used.