Starter spins but the engine won't crank

You press the button, the starter spins (sometimes with a flaring rev or a shrill metallic noise), but the engine isn't turned over and won't start. It's different from a starter that struggles or a weak battery: here, the starter spins freely. The most common mechanical cause is the starter clutch (sprag clutch / one-way clutch) slipping: this part no longer transmits rotation to the crankshaft. The starter pinion or its teeth may also be damaged (a grinding gear noise). More serious: if the timing has jumped (belt/chain) or the engine has lost all compression, the starter turns the crank but the engine can't run normally — these cases are for the professional. You tell them apart by the noise and the context. Don't keep hammering the starter: you risk worsening the wear on the starter clutch.