interjection
BAD : Please, if you cannot come, let me know as soon as possible.
GOOD : Please let me know as soon as possible if you cannot come.
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Please does not usually come immediately before a subordinate clause ('if you cannot come').
Note the usual positions: 'Please let me know as soon as you can.' 'Will you please let me know as soon as you can?' 'Will you let me know as soon as you can, please?'