What is an interrogative sentence?

An interrogative sentence is a question. An interrogative sentence always ends in a question mark.

Interrogative sentences are marked by the use of interrogative words (interrogative pronouns, interrogative adverbs, and interrogative determiners; e.g., “who,” “why”) or by a form of the auxiliary verb “do” (e.g., “Do you like me?”) or the main verb “be” (e.g., “Are you warm?”).

Note that an indirect question (e.g., “I wonder what he thinks”) does not count as an interrogative sentence and should not end in a question mark.