A. Short forms (I'm / don't / can't etc.)
I'm not tired.
I am.
Do you like tea?
No, I don't.
Yes, I do.
She isn’t tired but he is. (he is = he is tired)
He likes tea but she doesn’t. (she doesn’t = she doesn’t like tea)
In these examples, it is not necessary to repeat words (‘he is tired’, ‘she doesn’t like tea’).
am/is/are
was/were
have/has
do/does/did
can
will
might
must
• I haven't got a car but my sister has. (= my sister has got a car)
• A: Please help me.
B: I'm sorry. I can't. (= I can't help you)
• A: Are you tired?
B: I was, but I'm not now. (= I was tired but I'm not tired now)
• A: Do you think Ann will phone this evening?
B: She might. (= she might phone)
You cannot use 'm/'s/'ve etc. (short forms) in this way. You must use am/is/have etc.
• She isn't tired but he is. (not ... but he's)
• My sister has got a car but I haven't. (not ... but I haven't got)
• ‘Are you and Jim working tomorrow?’ ‘I am but Jim isn't.'
B. You can use I am / I'm not etc. after Yes and No:
• ‘Are you tired?’ ‘Yes, I am. / No, I'm not.’
• ‘Will Alan be here tomorrow?’ ‘Yes, he will. / No, he won't.’
• ‘Is there a bus to the airport?’ ‘Yes, there is. / No, there isn't.’
C. We use do/does for the present simple: (→ Units 6–7)
• I don't like hot weather but Sue does. (= Sue likes hot weather)
• Sue works hard but I don't. (= I don't work hard)
• ‘Do you enjoy your work?’ ‘Yes, I do.'
We use did for the past simple: (→ Unit 12)
• A: Did you and John enjoy the film?
B: I did but John didn't. (= I enjoyed it but John didn't enjoy it)
• ‘I enjoyed the film.’ ‘I did too.’ (= I enjoyed it too)
• ‘Did it rain yesterday?’ ‘No, it didn't.'
have you? / don't you? etc. → UNIT 40 | so am I / neither do I etc. → UNIT 41
Examples
I haven't got a car but my sister has.
I'm sorry. I can't.
I was tired but I'm not now.
She might.
Yes, I am. / No, I'm not.
Yes, he will. / No, he won't.
Sue does.
I did but John didn't.
Unit 39: I am / I don’t etc. • Full Interactive Textbook