choosing tense in the If-clause of a mixed conditional
If he had left half an hour ago ...
This means that I know for a fact that he did not leave half-an-hour ago. In other words it translates to:
If he had decided to leave half an hour ago (but he didn't) ... then he would be here.
If he left half an hour ago ...
This means that I allow the fact that he may have left half-an-hour ago.
If it is true that he left half an hour ago ... then he should be here. (maybe we just haven't met up with him yet)