Before you read a piece of fiction, you should

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establish a purpose for reading

Before you read a piece of fiction, you should.

"establish a purpose for reading."

Fiction can be a literature in the form of prose, that describes imaginary events or people. Also, it is a belief or statement that is false, but is often held to be true because it is beneficial to do so. That is why establishing a purpose for reading is needed.

This is a complete adjective clause.