In a grind the trucks — the metal axles — slide along the edge; in a slide it is the underside of the deck. The 50-50, both trucks on the ledge, is the entry point.
Mixing up the two words sends you hunting for the fault in the wrong place. A grind is metal on edge: in a 50-50 both trucks sit symmetrically on the ledge, which makes the balance the easiest to hold — the classic first grind. A boardslide instead has the underside of the deck sliding across the edge, body and board at right angles to your direction of travel, and that demands far more active balance. The most common mistake isn't the entry but the exit: jumping off in a panic instead of turning the board back into the direction of travel with a small weight shift as the ledge runs out. And one limitation that gets skipped — both need a reliable ollie first, because you have to jump onto the obstacle before anything can slide at all.