Stand MP into Down-Forward HP doesn't combo normally.
It will combo in counter hit automatically if stand MP counters.
But it will also normally combo if you hit on the second active frame of stand MP.

Give it a shot.

The way it works in this game is all attacks' frame advantage can be increased by hitting in later frames too. It's like how Dhalsim can make his slides safe: distance.
In this case, the first active frame of stand MP gives just barely enough hit stun to combo down forward HP; naturally a frame trapping target combo although unsafe.
Counter hits give a set additional amount of frames per attack type. Mediums get X amount of extra frames, heavies get Y amount of extra frames, and crush counters give more than enough to do anything you want.
The second active frame of stand MP hits and gives just enough hit stun to cover the down forward HP to link into a real combo.