It is hard to say what the definitively most difficult part of learning to write Chinese letters, although a common challenge people usually face when learning Mandarin is that the symbols don't give a hint as to how the word is actually going to be pronounced, but mostly just what the underlying meaning of the word is going to be. This together with the fact that Mandarin does not use either spaces or any other character to distinguish where one word ends and another word begins, means that you will need to memorize a lot of things while learning.
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