past progressive/continuous + simple past We use the past continuous / past progressive to say that something was in progress (happening/going on) around a particular past time. We also use the past continuous / past progressive together with a another verb in the simple past tense. The simple past tense verb refers to an action that interrupted or happened in the middle of the longer action (the past continous action). I was walking home when I heard the thunder. We were talking when/while we were driving home. I saw the building was burning as I approached. It happened while I was visiting Lijiang. I stayed in Lijiang for two weeks while I was visiting China. As I was walking I saw the crowd of people. (I saw a crowd of people as I was walking.) The phone rang as I was having dinner. When I woke up this morning the sun was shining and the birds were singing (The sun was shining and the birds were singing when I woke up this morning). source: Practical English Usage, Michael Swan, Oxford University Press

"As I was driving yesterday, I got a call on my mobile phone." This is correct, because it shows things happening at the same time. You could say the same thing using "while," but not "when." "When" means it is finished, and takes the simple past tense (e.g., "While I was living in Spain" vs. "When I lived in Spain").

It's a bear, isn't it, Richard! ;-) Again, in your example, you say, "When I wrote this comment..." (not, "When I was writing this comment"). The way you're using "when" is absolutely correct grammatically... my point was merely that "when" is used when the main verb that follows in the clause is in the simple past tense. Whereas "while" and "as" are best when the main subsequent verb is in the gerund form.

Brad - I see the logic in your comment. Then again, I'm not sure that logic alone can always be applied to understand our complex native language. So, I consulted the thesaurus and found that "when" and "while" are both conjunctions, and mutuallly synonymous. Both words had defintions such as: at the same time, during, immediately upon, just after, just as, meanwhile, and while. Nonetheless, I do see the reasoning behind asserting that "when" is related to a singular event and/or a point in time. It is my undersatnding (bearing in mind that I'm not an English teacher) that some subordinating conjunctions may be used at the beginning of an indepent or a dependent clause, and the order of the clauses is not important. When I wrote this comment I was debating with myself / I was debating with myself when I wrote this comment. (the conjunction "while" does seem more appropriate, but if they are synonymous then ... And I'm still debating with myself ... I think I need some divine intervention at this point.

I agree with Richard and Angela, but I think they're using "when" in a different way than I was. In their examples, the verb following the "when" isn't continuous/progressive, it's past tense (which is then followed by a continuous verb). So the example "When I woke up the sun was shining" isn't talking about when the sun was shining, it's talking about when I woke up. To use the past continuous, it would need to be, "When I was waking up, the sun was shining," which isn't correct. In their examples, "when" is used to established a sequence of events (first this happened, then that happened; or, when this happened, that was happening at the same time). If the two events are simultaneous, we use "while" or "as."