Are tightly and tight interchangeble as a verb?Are all of the sentences correct?What's the difference between the first and second sentence?

He held on tightly to her arm.
He held her arm tightly.
He held her arm tight.
My suitcase was packed tight.
My suitcase was packed tightly.

Question

Are tightly and tight interchangeble as a verb?Are all of the sentences correct?What's the difference between the first and second sentence?

He held on tightly to her arm.
He held her arm tightly.
He held her arm tight.
My suitcase was packed tight. 
My suitcase was packed tightly.

Answer

Sometimes we use the adjective form (tight) as the adverb (tightly).

"He held his hand tight on my arm."
In the US at least, this sounds natural and many native speakers would say it, even though it is (prescriptively) grammatically incorrect.

This works for other adjective/adverb pairs as well.

Examples:
She spoke nice and slow. (slowly)
Don't treat her bad. (badly)

Answer

They can be used interchangeably, but tight is used more informally. Tightly sounds more formal.

Answer

Tightly is an adverb.
Tight is an adjective.

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