Does not express a complete thought what kind of sentence?

A sentence that does not express a complete thought is called a fragment.

Here's why:

* Sentences need a subject (who or what is doing the action) and a verb (the action itself) to convey a complete thought.

* Fragments are missing either the subject, the verb, or both, making them incomplete and unable to stand alone as a sentence.

Example:

* Fragment: Running down the street.

* Complete Sentence: The dog was running down the street.

Let me know if you'd like more examples or want to discuss other types of sentences!