I'm wondering which one of these expressions is correct?
This stuff or these stuff?
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Stuff is a collective noun — it represents a group of objects. Just as one would say "this group" or "this pile," one would say "this stuff".
Stuff has a similar problem to the words content and land. they are already plural but we sometimes want to treat them as singular. E.g
Alf sold some stuff at auction.
Ben also sold some stuff.
Charlie also sold some stuff.
Dan didnt sell all his stuff.
We wish to say "3 out of 4 loads of stuff were sold at auction". Here the prefix "loads of" converts stuff to a singular collective noun phrase. "plot of" works for land. Not sure what works for "content". Some words (usually animals) already have collective nouns. E.g if stuff was sheep then we can say that "3 out of 4 flocks were sold at auction".
In your expressions, stuff is a noun and is uncountable. I would say it is "this stuff".
The only time I can think of stuff being plural is when it is a verb—as in he stuffs the turkey with ….