Is had you taken the test before a correct sentence?

No, "Had you taken the test before" is not a complete sentence. It's a dependent clause, meaning it needs an independent clause to make a complete thought.

Here are a few ways to make it a complete sentence:

* Add an independent clause: "Had you taken the test before, you would have known the answers."

* Change it into a question: "Had you taken the test before?"

* Add a subject and verb: "You had taken the test before."

Let me know if you'd like more examples!