What happens if u have an artifact that you've turned into a creature and given it persist and it goes to the graveyard? It comes back as an artifact, but does it retain the -1/-1 counter if it's not a creature?
Yes. Counters are not just for creatures see {raging ravine}.
So raging ravine would keep the +1/+1 until that land/creature was destroyed?
If i destroy an artifact or land that became a creature for a turn, it stays in the graveyard right?
in general, unless they are able to regenerate or blink it, or give it hexproof, if an effect says destroy something, be it artifact, land or creature, unless it has indestructable, it will be sent to the graveyard. not just creatures can be destroyed. making a land into a creature until eot doesnt change anything. {armageddon} destroys all lands, regardless if they are also creatures, see {dryad arbor}.