The man who you should marry is th eone who gave me the book.
Hello Guest

I believe Komountain's comment relates to the first 'who' in your sentence, so I'll comment on the 2nd.

For this 'who', you use 'who':

'...who gave me the book'.

This is because it's the subject of the verb 'gave'.

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In an exam setting, you should choose 'whom,' not 'who.'
That's what the traditional grammar says. However, 'who' is sometimes
used in less formal writings. I consider it as substandard, so I'd advise you to avoid it.
 MrPedantic's reply was promoted to an answer.
Sorry, I didn't know there were two 'who's.'
I stopped reading in the middle. That's why I got
low marks from the tests I took when I was a student.
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 CalifJim's reply was promoted to an answer.
Ha ha ha. I guffaw.

I hope you have some salt.