A relative clause is a type of dependent clause that provides additional information about a noun in the main (independent) clause. It is usually introduced by a relative pronoun such as "who," "whom," "whose," "which," or "that."
Let's analyze your phrases:
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"I closed that does not form a sentence": This phrase does not seem to form a coherent idea. It doesn't represent a complete thought and lacks proper structure.
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"I closed that modifies a noun in the independent clause": This seems to describe a scenario where a relative clause is modifying a noun in an independent clause. For example: "The book that I closed was interesting." Here, "that I closed" is a relative clause modifying "the book."
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"I closed that forms a sentence": Similar to the first point, this phrase is structurally unclear. If you meant to indicate that the relative clause could stand alone as a complete sentence, that would not typically apply to relative clauses since they are dependent.
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"I close it modifies a verb in the independent clause": This is an incorrect characterization of a relative clause. A relative clause typically modifies a noun, not a verb. For example, in "The man who closed the door is my neighbor," "who closed the door" modifies "the man."
In summary, a relative clause modifies a noun in the independent clause and is dependent on the main clause for its meaning. It cannot stand alone as a complete sentence.