Is what a sentence?

The phrase "what a" is not a complete sentence. It's a phrase that acts as an exclamation or an adjective phrase.

To make it a sentence, you need to add a noun and a verb. For example:

* What a beautiful day! (Exclamation)

* What a wonderful book this is! (Adjective phrase)

Let me know if you'd like to see other examples or have another phrase you'd like to check!