He fell off the ladder while he was changing a light bulb

[keannu]

This is a practice question of a grammar book, and I think either "when" or "while" suits it. Does it have to be only "while" here? I think "while" is usually for two simultaneous actions, and "when" is for a background action, but I can't tell the difference exactly.

gz194
ex)He fell off the ladder while(when?) he was changing a light bulb.

 

[William Jones]

Do you have to choose either 'when' or 'while?' Neither is wrong, necessarily, nor is either right. Both 'when' and 'while' are conjunctions and can combine the two sentences. 'While' does, as you already have indicated, express a simultaneous action, and 'when' indicates a time when something occurred, so there is a shade of difference between the two. I would, though, say that either could serve the purpose equally well. It is just up to you as to how you want the sentence to flow.