Si clause

Question

The phrase “if you could run” uses L’imparfait. But the phrase “if you run” uses the present tense. Could you explain this difference to me? Thanks.

Accepted Answer

So "if I could run" is a hypothetical case (#2). Therefore:
Si je pouvais courir. 
The other two cases are:
Si je peux courir. -- If I can run  
Si j'avais pu courir. -- If I could have run (If I had been able to run).

Answer

There are three levels of "probability" with si-clauses:
If I run. -- Si je cours. (Possible)
If I ran. -- Si je courais. (Hypothetical)
If I had run. -- Si j'avais couru. (Impossible, it already happened.)
This is pretty much parallel what you would use in English.