PEU. 256 and 426 says that
In most subordinate clauses, we use past tenses to express 'unreal' or conditional ideas:
If you asked me nicely, I might get you a drink.(second conditional)
But why not "If you had asked me nicely, I would have got you a drink."?(third conditional) It seems like one asked the other in the past. And what is the difference between those two sentences?