Could I use morphling's -1/+1 ability over and over to make it a 0/8 and then use the +1/-1 ability to bring it down to a 4/4? or a 5/3? or (given enough mana) whatever I want?

Nianque wrote:Could I use morphling's -1/+1 ability over and over to make it a 0/8 and then use the +1/-1 ability to bring it down to a 4/4? or a 5/3? or (given enough mana) whatever I want?

That would be cool! But no, unfortunately (or I guess fortunately, if you're the opponent).

Short answer: Although a creature can't deal negative damage in combat and instead deals zero, its power CAN be negative for the purposes of calculation. That means Morphling would in reality be a -2/8 (that deals zero damage), so the third ability will add +X to -2. Thus, the abilities are inverse operations, that is, they undo each other.

For a slightly longer answer that references the actual official rules, see comments #2 and #3 of this post.