Rule: The progressive is used to indicate a period of time during which an event (expressed by a non-progressive tense) occurs.
Examples:
(1) When his mother called for him to come in for supper, she noticed that he did not play with his friends any more.
(2) When I last saw him, he wore a red pullover.
(3) He walked across the street when a car ran into him.
(l)-(3) illustrate the time-frame function of the present progressive (or "Inzidenzschema"--cf. Nehls 1978:§4.3.11). In such cases the verb in the progressive expresses a predication that extends over a period of time (the time frame) during which another event or action occurs. With when clauses, if the main verb is not in the progressive, i.e. if the verbs in both subordinate and main clause are in the simple past, this usually implies a sequential and often a causal relationship. In (1), for example, with all three verbs in the simple past, the implication is that he stopped playing after or as a result of his mother's having called him; the intended meaning, however, is that at the moment when she called him, (or noticed him), he was no longer playing. The state of 'not playing', then, forms the time frame during which the calling and the noticing take place. Similarly, in (3), he did not walk across the street after the car ran into him, which would be the unlikely implication of both verbs in the simple past, but was in the process of walking when the accident occurred.
Compare (l)-(3) with the following examples, where two simple past tense verbs indicate a sequential relation between two events:
When she noticed me, she stopped talking.
When he saw me, he turned and ran.
He walked across the street when the light turned green.
When in these sentences is equivalent to after, though after, by emphasizing the temporal aspect of the relationship between the two clauses, tends to reduce the implication of a causal relationships. This is particularly true if the temporal clause has final position, e.g.:
The thief ran away when/after the police came.
When definitely implies a causal relation (i.e. he ran away because the police came), but after leaves the question open. Use of the past perfect instead of the simple past can have a similar effect: by emphasizing the temporal sequence, the implication of causation is weakened:
When the police had come, the thief ran away.
On the other hand, this sentence can also imply that the thief purposely waited for the police to come, and then ran away--a slightly different causal relation.
A sequential relationship between the two clauses is not necessarily implied if when means whenever, e.g.:
When(ever) she talked, I listened.
When(ever) she phoned, I pretended not to be home.
In this case the contemporaneity (as in the first sentence) or the sequentiality (as in the second) of the predications in the two clauses must be determined by the sense of the sentence as a whole. But here too, the special situation of a time-frame during which something happens--even if it is habitual--is expressed by a combination of the past progressive and the simple past:
When(ever) she phoned, I was (inevitably) taking a bath.