When and while

'While' carries a sense of the duration of an activity or event, and often goes with the present participle of the following verb.

'When' may carry such a sense, but may also refer to a specific point in time.

Examples of 'while':

'She arrived while I was sweeping the floor.'
'Be careful while you are carrying that tray.'
'While you were singing, I went to the kitchen.'

'When' could replace 'while' in those examples, though it would be less natural.


Examples of 'when':

'She arrived when the TV program had just started.'
'Be careful when you pick that knife up.'
'When you began to sing, I went to the kitchen.'

'While' could not replace 'when' in those examples.

Mary was talking on the phone when her mother came home
We were watching TV when Mary went out with her friend.


While Mary was talking on the phone, her mother came home.
While we were watching TV, Mary went out with her friend.

We were watching TV while our brother was playing the online games
Mary was talking on the phone while her mother was cooking in the kitchen.

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http://iamlearningenglish.blogspot.hk/2005/09/when-versus-while.html

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