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Is this OK:

I haven’t had feelings this strongly for someone in a long time.

And

I haven’t had feelings this strong for someone in a long time.

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Use #2.

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In #2, it's the feeling that is strong. The adjective 'strong' modifies the noun 'feelings'. It's an easy and simple meaning.

In #1, the adverb 'strongly' modifies the verb. It means that it is the having that is strong. This is an awkward thing to say.

Clive

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Comments  
Clive

Use #2.

Why?

#1 is grammatically incorrect.

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Rover_KE

#1 is grammatically incorrect.

Can you tell me why?
 Clive's reply was promoted to an answer.