Tag questions

A question tag (also known as question tail) is a grammatical structure in which a declarative or an imperative statement is turned into a question by the addition of an interrogative fragment (the "tag"). For example, in the sentence "You're John, aren't you?", the statement "You're John" is turned into a question by the tag "aren't you".^[1

The English tag question is made up of an auxiliary verb and a pronoun. The auxiliary must agree with the tense, aspect and modality of the verb in the preceding sentence. If the verb is in the present perfect, for example, the tag question uses has or have; if the verb is in a present progressive form, the tag is formed with am, are, is; if the verb is in a tense which does not normally use an auxiliary, like the present simple, the auxiliary is taken from the emphatic do form; and if the sentence has a modal auxiliary, this is echoed in the tag:

• He has read this book, hasn't he?
• He read this book, didn't he?
• He's reading this book, isn't he?
• He reads a lot of books, doesn't he?
• He'll read this book, won't he?
• He should read this book, shouldn't he?
• He can read this book, can't he?
• He'd read this book, wouldn't he?
• He'd read this book, hadn't he?

A special case occurs when the main verb is to be in a simple tense. Here the tag question repeats the main verb, not an auxiliary:

• This is a book, isn't it?