Any difference between things and stuff?

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In meaning, no not really. Grammatically though "things" is a plural noun and countable, but "stuff" is a non-count noun.

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Yes has , you can count things , but cannot  count stuff .Stuff is an uncountable noun you can only measure .

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'things' is normal but 'stuff' is very casual;
not really used in polite situations but very common with friends and family/normal people, ha ha.

:)

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Yes, "stuff" is said by people who like to boast.

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Here is a good explanation:

https://dictionary.cambridge.org/amp/british-grammar/uncountable-nouns/thing-and-stuff