... than if you had started from directly below

[KJOU]

If you want to use the inclined plane to help you move an object (and who wouldn’t?), then you have to move the object over a longer distance to get to the desired height than if you had started from directly below and moved upward. This is probably already clear to you from a lifetime of stair climbing.

I don't know why the past perfect construction is there instead of "started" or "have started."

Is it "subjunctive" or " just conditional" ?

 

[tedmc]

It is a third conditional, which is about something that didn't happen (hypothetical).