It all depends on the types of activities -continuous or moments in time. Examples 1, 2 and 4 all have one continuous activity (driving, reading and buying) and one moment in time (saw, rang and saw). For all three of these examples, you could reverse the order of the activities in the sentence and switch the While/When ie. I saw Tim WHILE I was driving....The bell rang WHILE I was reading...I was buying books WHEN I saw a friend. For example 3, you can use either word since they're both continuous activities: ....was raining WHILE I was riding OR was raining WHEN I was riding OR I was riding WHILE it was raining OR I was riding WHEN it was raining. All depends on how you structure the sentence. Does this help?