Examples of Subject-Auxiliary Inversion

In English Subject-Auxiliary Inversion (also called Subject-Aux Inversion or SAI) is the movement of an auxiliary verb to sentence-initial position (preceding the subject) to form a question.

Main verbs in English cannot undergo SAI, if we try to invert the main verb and the subject, we get a completely ungrammatical sentence in English (though the order is perfectly grammatical in some languages).

If a declarative sentenceOpens in new window contains an auxiliary verbOpens in new window such as have or be, a modal auxiliary Opens in new windowlike may or could, or the copular form of be, a yes/no question is created from the sentenceOpens in new window by applying the rule of Subject-Auxiliary Inversion, i.e., switching the position of the subject and the verbal element that follows it.

The sentences labeled “b” in 1) through 6) are yes/no questions that result from the application of subject-aux inversion to the declarative sentences labeled “a”.

Notice that in each case, the position of the verbal element that follows it have been reversed to create a question. The labels on the right describe the verb in boldface in each sentence.

For declarative sentencesOpens in new window that do not have an auxiliary verbOpens in new window, a modalOpens in new window, or copular be, the rule of subject-aux inversion is not applied to form a yes/no question. Instead, an appropriate form of the auxiliary verb do is placed at the beginning of the sentence.

This process of adding do to a sentence is referred to as do insertion or do support. The auxiliary do allows the speaker to express tense differences, as 7a) and 7b) illustrate.

StatementYes/No QuestionTense
7a)  He runs every day.Does he run every day?simple present tense
7b)  He ran every day.Did he run every day?simple past tense

In British English, sentences with the main verb have (not the auxiliary verb have) also undergo subject-aux inversion to form yes/no questions, as shown in 8b). But in American English, do insertion is used instead, as shown in 8c). This is an important difference that learners of English should know.