Is How quickly you learn a complete sentence?

"How quickly you learn" is not a complete sentence. It's a phrase that expresses a question, but it lacks a subject and a verb.

Here are some ways to make it a complete sentence:

* "How quickly do you learn?" (This is a question about someone's learning speed)

* "It's amazing how quickly you learn." (This is a statement about someone's learning speed)

* "You learn very quickly." (This is a statement about someone's learning speed)