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When I try to do this one spell it says "Doesn't fully understand this spell" does this mean I'll never be able to do it, or can I begin to understand it after I become a bettter mage

It basically means that you are too stupid to understand how it works.

so will I ever be apply to get my int. up enough to do it or am I stuck with my stiupedty forever

You are born with your (Characters) lack of Inteligence, and so you will die with it.

Stupidity forever, my friend.
Never underestimate the power of the ignorami tho', there are so many of them that if they ever got the idea of banding together, tey'd prolly be quite powerful. Not that that would ever happen tho

this has been discussed before, use the search option next time

Don't worry, though. Although you'll never be able to cast that spell, what you lack in intelligence could be made up by your combat strength. I can understand every spell, but my fighting isn't as good.