Interrogative sentences ask questions and have two main types: yes/no questions answered with yes or no ("Do you like pizza?") and wh-questions seeking information ("What did you eat?"). Questions typically involve special word order, question words, and intonation patterns that distinguish them from statements.
Practice converting statements to yes/no questions by identifying and moving the auxiliary verb. For wh-questions, start with the question word, then add the auxiliary if needed, then follow with subject and main verb.
You already know that every sentence has a subject (who or what the sentence is about) and a predicate (what the subject does or is). In declarative sentences, the subject comes first: "She *is* happy." Interrogative sentences — questions — disrupt this default order. The most important structural operation in English question formation is subject-auxiliary inversion: the auxiliary verb moves to a position before the subject, signaling to the listener that a question is being asked rather than a statement being made.
For yes/no questions, the process is straightforward once you spot the auxiliary. Take "She is happy." The auxiliary *is* inverts with the subject: "Is she happy?" Take "They will arrive soon." Inversion gives "Will they arrive soon?" The challenge arises when the declarative sentence has no auxiliary — "She likes tea." English does not allow bare inversion of a main verb (*"Likes she tea?" is ungrammatical), so it inserts the dummy auxiliary do to carry the tense: "Does she like tea?" This is why your prerequisite knowledge of auxiliary verbs is essential here: you must recognize which element in the sentence is the auxiliary, because that is the element that moves.
Wh-questions add a second layer. They begin with a question word — *who, what, where, when, why, how* — that names the type of information sought. The question word occupies the front position, and subject-auxiliary inversion still applies to the rest of the sentence: "What did she eat?" = [what] + [did ← inverted auxiliary] + [she ← subject] + [eat ← main verb]. When the question word *is* the subject, inversion disappears entirely — "Who called you?" not "Who did call you?" — because moving the auxiliary would require it to follow its own subject, which is already at the front.
Intonation completes the picture. In speech, yes/no questions typically end with rising pitch, signaling incompleteness and inviting a response. Many wh-questions use falling pitch, since the question word already signals that information is expected. In writing, the question mark does the same work. Together, word order, auxiliary selection, and intonation form an interconnected system: change one component without the others and the result sounds strange or ambiguous. Mastering question formation means understanding these three components not as separate rules but as a coordinated grammatical package.