Will crank but wont start?

Make sure battery has a full charge and connections all good. Have a helper crank it while you check for spark at the plugs, gasoline engine. If spark everywhere, use a gage and check proper fuel pressure and fuel injector pulse. Any applicable trouble codes? Those are the basic checks, in my opinion. sometimes, one of the circuits, ignition--fuel--whatever, may have a voltage drop for some reason other than the battery or connections. In that case, jumpers can help. Engine cranking all your systems get battery voltage, engine running, the systems get generator voltage, and that can be several volt difference.

If it starts when jumped it is likely the starter drawing too much current or one of the battery or starter cables.