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Hello,

1.She can both speak and write Japanese.

2. She can not only speak but also write Japanese.

Sometimes the conjuctions both ....... and , not only ........but also seem similar in usage.

But there may be some differences between two.

Could anybody explain their usage?
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The not only...but also correlative conjunctions add a certain frisson of surprise to the second term, stressing it as a relatively unexpected addition. Both...and simply sets up the two terms in parallel.
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<Both...and simply sets up the two terms in parallel.>

If there is stress on "and write", it can do the same as not only...but, IMO.