When is what part of speech?

"Is" is a verb. Specifically, it's a linking verb (also called a copula) that connects the subject of a sentence to a predicate nominative or predicate adjective.

Here's a breakdown:

* Verb: A word that describes an action, state of being, or occurrence.

* Linking verb: Connects the subject of a sentence to a word or phrase that describes or renames the subject.

For example:

* "The sky is blue." (Here, "is" links the subject "sky" to the predicate adjective "blue".)

* "She is a doctor." (Here, "is" links the subject "she" to the predicate nominative "doctor".)