Engine runs -blade engages and throws grass, won't drive

Make sure all of the mounting connections are good and bolts are tight. Make sure the spindles are not at an angle causing the blades to be uneven.

Engine off
I would check the tension, still taught?
Anything caught in mower blades? Causing a locked deck?
Your belt worn out is most likely if none above.
PTO Clutch possibly bad. Runs for a second(turns blades) then throws belt. Good luck.

The gear would, obviously, have been engaged when you hit the root. I presume this will have caused some resistance in the friction material of your transmission. Consider the engine rotating the trans and the root stopping it from rotating, the transmission and the blade will have borne the brunt . Have your transmission checked for damage

Your belt may be on wrong or slipping.

Sounds like a broken shear pin in the drive pulley. Check the belts and see if they are slipping. Broken shear pin will not let the blades spin under pressure. .

sounds like he bent a blade that's when the belt went

check your blades to make sure that they are not hitting the deck,, this would cause the knocking sound

Most likely what you did was burn one or more spots on the belt. When the blades and belt stop turning, like in tall grass, the engine doesn't stop and will burn flat spots on the belt at the engine pulley. Every time the flat spot hits a pulley, it will jump and can set up a bad vibration. Run your hand around the sides of the belt or take it off and you will feel or see the thin spot(s) on it. Replace belt.

Sounds like the blades are dull. HAve them sharpened

It sounds to me like a very loose belt or very worn out belt and also an adjustment problem.