adverb
BAD : Putting fluoride in the water may solve the problem partly.
GOOD : Putting fluoride in the water may solve the problem to some extent.
GOOD : Putting fluoride in the water may partly solve the problem.
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At the end of a clause use to some extent/to a certain extent or to some degree/to a (certain) degree (NOT partly ). Compare: 'I partly agree with him.' 'I agree with him to some extent.'