A. in the train; in the sun (sunshine); in the shade; in the dark; in bed weather etc.
A1. (write) in ink; in biro; in pencil; (write) in words; in figures; in BLOCK CAPITALS etc.
A2. (be/fall) in love (with somebody)
A3. in (my opinion)
B. At the age of ~ etc. We say ‘at the age of 10; at 100 miles; an hour; at 0 degrees etc.
C. On holiday/ on a tour etc. We say (be/go) on holiday/ on business/ on a trip/ on a tour/ on a cruise etc. We can also say ‘go to a place for a holiday/ for my holiday (s)‘
C1. on television/ on the radio; on the phone/ telephone; (be/ go) on strike; (be/go) on a diet; (be) on fire; on the whole (= in general); on purpose (= intentionally)