I used to write.
It means that, in the past, you wrote many times. But you don't write any more now.
I'm used to writing.
It means that you are accustomed to writing. It is natural for you. You write many times now.
2014년 1월 3일
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Thank you!
2014년 1월 4일
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What is your question?
2014년 1월 3일
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I used to write = when I was a child, but now I don,t write ..for example .. I used to write my homeworks when I was a pupil. Now I,m not the pupil anymore.