got this atv given to me starts and runs but only in reverse when i try to shift into 1st it grinds and wont engage but when it is off it will cycle thru all the gears just fine i pulled it down the road behind a truck while off and the gears work it is just when it is running it wont go into first what do i need to fix

 

What you are describing COULD actually be the rear gearbox's bearings have gone out, and not your transmission - we've seen this kind of problem several times over the years. With a ring & pinion gear system, when in reverse you are applying force in the opposite direction, so the gears can still mesh - when in forward, the pinion gear pushes away from the ring gear.