Example:
You want a bike, don’t you?
We are using dummy ‘do’ in this tag question because there is no auxiliary verb in this sentence. So we are adding an auxiliary as well as ‘not’ because the tag question usually has to be opposite to the actual sentence (if the question is positive, the tag question will be negative).
Similarly,
We want freedom, don’t we?
She likes to play, doesn’t she?
To answer a question by using ‘yes/no’
When we need to answer shortly in ‘yes’ or ‘no’, we can use the dummy operator or the modal verb or the auxiliary to answer.
Example:
Are you busy?
-Yes, I do.
Here, the verb ‘be’ doesn’t take the ‘do’ operator’, so this reply is wrong. We can reply
-Yes, I am. (Positive)
-No, I am not. (Negative)
Similarly,
Have you a car?
-Yes, I have. (Positive)
-No, I don’t have. (Negative)
Do you want a pen?
-Yes, I do.
-No, I don’t.
‘Tense inflection’ and ‘Third person inflection’
What’s special about ‘do’ is that it takes the ‘tense inflection’ and the ‘third person inflection’.
Example:
He has a sister.
-he have a sister (this is wrong because ‘he’ is third person singular number).
So we should say
-Does he have a sister?
So only one of a verb the ‘do’ takes a person inflection.
He doesn’t have a sister.
-He doesn’t has a sister. (Wrong)
-He don’t has a sister. (Wrong)
When we say a sentence in the past tense
He had a sister.
-Did he have a sister? (Question)
Here, we put the tense inflection on the ‘do’. So ‘do’ becomes ‘did’ and ‘has’ becomes ‘have’.
He had a sister.
-He didn’t have a sister. (Negative)