The “a” in Helvetica has a tail; Arial does not. Also, the bowl of the “a” flows into the stem like a backwards “s”; the bowl of Arial’s “a” simply intersects the stem with a slight curve. (Interestingly, the Grotesque “a” has a tail, just like Helvetica. The bolder weights of Helvetica have no tails, an inconsistency that bothers some people. Maybe it bothered Monotype, too.) Arial’s “a” has always seemed a little badly drawn to me, but maybe it’s just me.