The difference between A great deal and A thing or two
When used as nouns, a great deal means a large amount, whereas a thing or two means a considerable amount.
A great deal is also adverb with the meaning: very much.
check bellow for the other definitions of A great deal and A thing or two
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A great deal as an adverb (idiomatic):
Very much; to a great extent; a lot; lots.
Examples:
"We had a great deal more money afterwards."
"They had a great deal less than we had."
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A great deal as a noun (idiomatic):
A large amount; a lot.
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A thing or two as a noun:
A considerable amount; a lot.
Examples:
"He's a good mechanic, and surely knows a thing or two about car engines."