We use both when and while as subordinating conjunctions to introduce adverbial clauses of time. They mean during the time that and indicate that something is or was happening when something else occurred:

• The prisoners escaped when / while the prison warders were eating their lunch.
  
• When / While the prison warders were eating their lunch, the prisoners escaped.

Note that we can also use as and whilst in the same way, although they sometimes sounds more formal or literary

• As the sun went down, I sipped a rum and coke on the balcony.
  
• I sipped a rum and coke on the balcony whilst the sun went slowly down on the horizon.

Note that during, which also introduces a longer period of time, is a preposition which is use with a noun or noun phrase:

• I first met my future wife during my stay in Casablanca.
  
• I first met my future wife while I was staying in Casablanca.

when not while

We use when, not while, to talk about something that occurs at the same time as a longer action or event that is described in the main clause:

• I was asleep in my chair when Dora rang to say she wasn't coming home.
  
• We were playing monopoly when the lights went off.

We also use when, not while, to talk about one event that happens immediately after another and to talk about periods of time in the past.

• When the lights went out, everybody groaned: "Oh no, not another power cut!"
  
• When I was a little boy, power cuts were very frequent, but that was just after the war.

When can also be used instead of whenever, meaning every time that:

• I always visit my mother-in-law when I'm in Manchester.
  
• I always visit my mother-in-law whenever I'm in Manchester.

We often prefer while to when to describe the longer action of two events or to talk about two longer actions that go on simultaneously:

• Dora left a message on the voice mail while I was asleep in the chair.
  
• While I was writing my Christmas cards, the children were decorating the tree.
  
• I cooked the supper while Jenny did the ironing.

Note from the above examples that while a progressive tense is normally used to describe the longer action associated with a while time clause, simple tenses are also possible.

Note also that it is often possible to omit subject + be in when- and while-clauses if the main and subordinate clauses refer to the same subject:

• When (you are) crossing the road, be careful to look right, left and right again.
  
• They came across human remains while (they were) excavating the site.
  

While is not used only used to introduce adverbial clauses of time. In more formal usage, it is used to link or balance ideas that contrast each other:

• While I am happy for us all to eat at home, I don't want to spend hours in the kitchen preparing the food.
  
• While the news from the front has so far been good, there will almost certainly be days when we must expect heavy casualties.

Note in this usage the while-clause is normally placed as the first of the contrasting points.

meanwhile = during this time

Meanwhile, meaning during this time, is a linking adverb which connects and contrasts ideas between two sentences. It indicates that one event is going on at the same time as another:

• Slice and brush the aubergines with oil and bake in the oven till soft. Meanwhile, melt some butter in a small pan…
  
• Why don't you prepare the boats ready for the water?Meanwhile, I'll check to see that we've got enough oars.
  
  

a while = a short time

Note that when while functions as a noun, it is nearly always used with an indefinite article:

• I haven't seen you around for a while. Where have you been?
  
• Let's just wait a little while longer. He's bound to turn up eventually.

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