In short replies to affirmative statements, we use why?
In short replies to negative statements, we use why not?
Compare:
- ‘Jane has decided to quit her job.’ ‘Why?’ (NOT Why not?)
- ‘Jane didn’t like the proposal.’ ‘Why not?’ (More natural than ‘Why?’)
The structure ‘why should’ can be used to express surprise.
- I wonder why anyone should want to do something like that.
This structure can also be used to suggest anger or refusal.
- ‘Lend me a pound.’ ‘Why should I?’
Why can be followed by an infinitive without to. This structure is used to suggest that an action is unnecessary.
- ‘Why wait for him? He will not come.’ (NOT Why waiting for him?) (NOT Why to wait for him?)
‘Why not + infinitive without to’ can be used to make suggestions.
- ‘Why not arrange a party in his honor?’ (This is a polite way of saying: I think we should arrange a party in his honor.)
- ‘Why not buy now and pay later?’
- ‘Why not make it up with him?’
- ‘Why not consult a specialist?’
‘Why don’t…’ can be used in the same way.
- ‘Why don’t we arrange a party in his honor?’ (NOT Why don’t arrange a party in his honor?)
- ‘Why don’t you make it up with him?’ (NOT Why don’t make it up with him?)