Blade is loose, turning nut just turns motor.

Before you start removing covers and panels unplug the mower. Turn the mower over and carefully try to spin the blade by hand. Your checking to where the issue is most likely to be.
Does the motor sound different when you pull the Start trigger? If it sounds a lot faster than normal, it could be an issue between the motor and gearbox.
Does the blade spin really easily and feel like it's not bolted down? Does the blade not turn at all like it's 'locked' or 'jammed'?
- A loose or floppy blade is commonly the centre mount bolt for the blade is loose or the blade isn't seated correctly.
- A 'locked' blade could be a gearbox or a motor issue.

are you turning nut the right way ! on a lot of mowers, the nut turns off the opposite way the blade turns, to keep it tight. good-day ! Loosen the nut in the center of the blade with a wrench by turning the nut counterclockwise.

Dependent upon the size of the shaft, a choice of suitable tools would vary. Considering this is the shaft off of an electric motor, may I suggest you wrap a rag around the area of the shaft where you can grip it and use a pair of Vise Grip locking pliars to hold it while you turn the nut. If the vise grips won't open wide enough, then use a rag with a pipe wrench locked onto the shaft to hold it.

The safest way to do this is to use a blade holder that you may be able to purchase from a parts store. It clamps to the side of your mower deck and keeps the blade from turning.

Here's the scoop on the MM875: it has a plastic piece called a "blade insulator" (part # 241381-01) that "locks" the shaft to the blade, so that you can remove the nut by stopping or blocking the blade. If it has been damaged or stripped out, then stopping the blade won't do anything. However, the metal spacer directly under the nut IS still locked to the shaft (because of its shape), so you can use a pipe wrench or something to hold that in place and unscrew the nut. You might even be able to get someone in a home improvement store to do this for you. But you will still need to replace the insulator, which is only a few dollars.

Block the blade with a piece of wood so it stops the motor and blade from turning while your loosening the nut.

I find that nut splitter work ok on occasion if you can get one in there, and just replace the nut(sounds like that's going to happen anyways regardless) , or pick it up, set the nut on a solid piece of steel, so that it's supporting the weight of everything, and hit the other side of the nut with a hammer, or a hammer and chisel, with the chisel perpendicular to the shaft and the blade splitting it the long way. Other tricks, are to heat it up red-hot, and hit it with water immediately, or even canned air inverted(will freeze it instantly) in order to get the surfaces to change enough to break loose. Hope these helped; they've been standbys for me for years.

IF IT IS A CORDED MOWER MAKE SURE IT IS NOT PLUGGED IN -
IF IT IS CORDLESS, MAKE SURE SAFETY DEVICE (KEY) IS REMOVED

If you have a work bench, place your mower on the bench on its side or upside down where you can see what you are doing. If no bench, the floor of your garage or your driveway will work too.
Get a small 2 x4 block, a C-Clamp or some other device to keep the blade from spinning.
Get a ratchet or breaker bar and socket to fit the bolt in the center of the blade.
After you are sure the blade is blocked from spinning, turn the socket and ratchet to the RIGHT (clockwise) to loosen the nut until you can completely remove it.
When you install the new blade, look at it to see if it is marked "grass side" if it is not marked, the sharpened area on the blade should have the "beveled" edge toward the housing and remember the nut will turn to the left (counter-clockwise) to install and tighten. (Electric mowers have LEFT HAND THREADS due to the rotation direction of the motor.)

If it's like mine (MM 875), it has a plastic "clutch" that has little edges on it that broke off when I was chopping sticks with the mower ;-) Went to the local B&D place and got a new one for less than $3.

But now I'm worrying about the nut holding the blade on -- if I try to get it good and tight, the plastic "clutch" spins around off-kilter... Funny how I never even looked at the blade after I bought the mower used, but never worried about the blade being loose and flying off.

i uses impact gun with 3/4 " socket.run it in reverse or you have to remove the stationary parts of the motor.mark them so u will know how to put them back together & somebody has to hold the armature will u r turning the nut on the blade.