New belt. New slip pads on driven pulley. The belt doesn’t stop moving so the transmission can be put in gear without grinding. Not mine. Trying to help an elderly man.

 

I had to slow the idle to get mine to engage without grinding. When I start it I have to let it idle for a few seconds until it slows down and then I have no problems. If this started just when the new belt was installed their may be a problem with the belt or pullies.