While Wookie58 is putting the plan together, I would like to get a clarification of terms, please.
You wrote that the starter motor is turning, but the engine will not crank, right?
Do you mean that the starter motor is turning freely, like there is no resistance and nothing else in the engine is moving, or do you mean that the starter motor is turning the crankshaft and related parts, but the engine refuses to start?
I know this may seem an odd question, but when trying to diagnose a problem remotely, it is crucial that we are all on the same page with terms.
If the starter motor is turning, but nothing else is moving, the starter clutch will be on the short list of possible culprits.
If the starter motor is indeed turning the crankshaft, then the starter clutch will not be on the list at all. The problem at this point will be electrical or fuel related.