Tom's decided to postpone his studies for a year.
1) Who has decided to postpone his studies for a year? (вопрос к полдежащему)
2) Tom's decided to postpone his studies for a year, hasn't he? (разделительный)
3) What has Tom done? (специальный, к сказуемому)
4) Has Tom decided to postpone his studies for a year or two years? (альтернативный)
5) What has Tom decided to postpone for a year? (специальный, к дополнению)
6) Has Tom decided to postpone his studies for a year? (общий вопрос)