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, ....