Why is “while” used in “He stopped briefly while driving” but ‘when’ in “When I first came to this city, she was working as a waiter”? Both sentences refer to moments

Question

Answer

Brief answer: 
"While" (adverb/conjunction) is normally used with an incomplete action, whereas "when" (adverb/conjunction) is normally used with a completed action.

Examples:
While I was working in the yard, it started to rain. So, I went inside.
I was working in the yard when it started to rain. So, I went inside.
When it started to rain, I was working in the yard. So, I went inside.
It started to rain while I was working in the yard. So, I went inside.

In these examples, while is roughly equivalent to "during the period that" and when to "at the moment that."

During the period that I was working in the yard, it started to rain. So, I went inside.
I was working in the yard at the moment that it started to rain. So, I went inside.

Other comments:
For native speakers, there is some overlap. The grammar books describe standard (mass-media) printed English, which is narrower than general spoken English.

Examples:
When I was a university student, ....
While I was a university student, ....

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