Avoidance of repetition
Auxiliary verbs are often used alone instead of full verbs. Study the following sentences.
X: Do you want it?
Y: Yes, I do. (= More natural than ‘Yes, I want it.’)
X: Did you see him?
Y: No, I didn’t. (= No, I didn’t see him.)
X: Who let the cat in?
Y: John did. (= John let the cat in.)
The same structures are possible with non-auxiliary be and have.
I knew she would be upset, and she was. (= I knew she would be upset, and she was upset.)
He says he hasn’t got a girlfriend, but I know he has. (= … I know he has a girlfriend.)
The addition of contraries to remarks already made
He used to smoke, but I never did.
She didn’t come, but you must.
You must go, but she needn’t.
Note that the negative of used to is never did or usedn’t to.
The negative of must is needn’t.
When can expresses permission, its negative is mustn’t.
When can expresses ability its negative is can’t.
‘Must I go?’ ‘You needn’t.’ (= No obligation to go.)
‘Can I go?’ ‘You mustn’t.’ (Prohibition)
Fill in the blanks with appropriate contrary additions.
1. He can smoke if he wants to, but you —————–
2. You needn’t wait here, but he ——————-
3. You must attend the party; your sister ———————
4. He can speak English well, but I ——————–
5. He may go now, but you ——————-