I understand that 「の」 can be used to indicate possession or to change a noun to a modifier for another noun. But neither of those seems to be the case here.
What usage of 「の」 is being used here?
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Sign up to join this community終わりのない is not a complete sentence. This phrase can be used as a relative clause which modifies another noun.
- 終わりのない話 a story which has no end; a never-ending story
- 終わりがない話 a story which has no end; a never-ending story
As you can see, this type of の is a subject marker (just like が) which can be used only in relative clauses.