If I do ... and If I did ...

If I do ...

• Lisa has lost her watch. She tells Sue:
  • Lisa: I've lost my watch. Have you seen it anywhere?
  • Sue: No, but if I find it, I'll tell you.

In this example, Sue feels there is a real possibility that she will find the watch.
So she says: if I find ..., I'll ...

If I did ...

• Joe says:
  • If I found a wallet in the street, I'd take it to the police station.

This is a different type of situation. Here, Joe doesn't expect to find a wallet in the street; he is imagining a situation that will probably not happen. So he says:
if I found ..., I'd (=I would) ... . (not if I find ..., I'll ...)

Meaning is not past.

• Sarah wants to phone Paul, but she can't do this because she doesn't know his number. She says:
  • If I knew his number, I would phone him.

Sarah says: If I knew his number ... . This tells us that she doesn't know his number. She is imagining the situation. The real situation is that she doesn't know his number.

When you imagine a situation like this, you use if + past. But meaning is present, not past.