What is the difference between "There will be users who doesn't buy something" and "There will be users who don't buy something"? Are they both grammatically correct?
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It's users, plural. The verb has to agree. Users buy X. Users don't buy Y. Users who don't buy Z. Saying "users who doesn't buy something" would be ungrammatical.
Edit: I took the "something" in your example to be a placeholder for a product name. If that is not the case, i.e. if what you actually want to say is that there will be users who will buy nothing at all, then, as JoseK points out, you should be using "anything" rather than "something". That's because anything is a negative polarity item, while something is a positive one. So, to sum it up:
Doesn't is 3rd person singular. Don't is everything else, including 3rd person plural. So "there will be users who doesn't buy something" is wrong and should be "there will be users who don't buy something" or "there will be at least one user who doesn't buy something"
The difference is singular vs. plural. It's the conjugation of "to do" that creates this distinction; "He does" vs. "They do":
Implies that "who" is singular (a he or she). Appropriate answers to "Who doesn't?" would be "He does", "Sally does", or even "I do". A plural "we don't" or "they don't" would still be appropriate but it will mean that the asker didn't ask the right question.
Implies that "who" is plural. The same applies here, but opposite. The obvious answer is "They don't". If you answer a singular, "He doesn't", it's actually a bit ambiguous whether he is the only one that doesn't, or if he's part of the group that doesn't.