When is followed by a clause with an activity that the first is a short interruption, or the second occurs at nearly the same time or shortly after the first activity. A nonprogressive verb form is more commonly (but not always) used for activities of short duration¹.
While is followed by a clause with a same-time (simultaneous) activity and includes a verb expressing duration². The while-clause often expresses a background activity to the focus-activity in the main clause.
Remember: Clauses are groups of words which have meaning, but are often not complete sentences. Some clauses begin with the word "when" such as "when she called" or "when it bit me." Other clauses begin with "while" such as "while she was sleeping" and "while he was surfing." When you talk about things in the past, "when" is most often followed by the verb tense Simple Past, whereas "while" is usually followed by Past Continuous. "While" expresses the idea of "during that time." Study the examples below. They have similar meanings, but they emphasize different parts of the sentence.
Examples:
- I was praying when she called.
- While I was praying, she called.
When has two meanings:
- 1) same time: He was working for an Internet start-up when I was still attending classes in college. (was attending —process)
- 2) immediately after, in sequence: I was attending classes when the war broke out, and I had to postpone my education. (broke out —punctual)
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