I am a little confused about using "when v-ing" and "while v-ing". When should I use "when v-ing" instead of "while v-ing" ?
1- Always keep some change with you. It’s useful when buying a bus ticket.
2- Always keep some change with you. It’s useful while buying a bus ticket. (The dictionary say this usage is wrong)
3- Be careful when crossing the road.
4- Be careful while crossing the road. (Is it correct to use while here? )
5- I used to read a book when using public transport.
6- I used to read a book while using public transport. (Again, is it correct to use while here?)