Hopping between l'imparfait and passé composé

Question

From writing challenge b2 week 62, this extract describes events in the past: 
La "navette" qui DEVAIT nous amener à l'hôtel ÉTAIT en fait un vieux bus de campagne cabossé dans lequel on A PU apprécier toutes les aspérités de la route."    Why doesn't it stick to l'imparfait for all 3 verbs?

Answer

Bonjour Joakim,
It seems to me that the use of the imparfait is for the descriptive parts of the phrase describing the bus itself while the passé composé explained a «one-time» past event while aboard the bus, that of being «on A PU apprécier toutes les aspérités de la route»  The manner that I learned the differentiation is:  the imparfait is used in describing the setting, events, etc. while the passé composé is used to explain a one-time event with a clearly defined beginning and end.   The following are the highlights from the lesson about l'Imparfait and Passé composé
Reminder of the different usages for L'Imparfait: expressing continuing actions , habits or repeated actions  or for descriptions and past states.  
Note that Le Passé Composé is more like a direct equivalent of the English Simple Past (I went, I did ...). 
It is used for past actions/events that happened once, with a clear beginning and end, as well as for a succession of actions in the past.
A past action that has a clear time-frame in the past, insisting on it being a single, whole past event.
In the example:  «La "navette" qui DEVAIT nous amener à l'hôtel ÉTAIT en fait un vieux bus de campagne cabossé»  is more a descriptive phrase.

J'espère que cela vous aidera.
Bonne chance,
Ron

Answer

Than you, Ron.  It is not easy to understand how these one-time events work.  I might be tempted to incorrectly write  'La "navette" qui A DÛ nous amener ' because that bus was only supposed to transfer them once and the journey it was supposed to make had a clear beginning and end.

Answer

I understand your sense exactly because I too have the same problem when writing and/or speaking and switching between the two; however, with your example, it was easier to review it and think about how there is a difference.  When I could finally «see» the nuances of the bus intended to take them to the hotel and the fact that it was an old bus.  To me, this then became an issue of description.  Now, having said all of that, it is possible that Aurélie or Gruff might have another take on it.
Je suis très content que j'ai pu vous aider.
Bonne chance,

Answer

Bonjour Joakim !Like Ron, I'd say that L'Imparfait tends to refer to actions that lasted, (hence its use for descriptions), that you consider in their development, whereas Le Passé Composé is more about punctual, brief actions considered as a whole.Let's look at this sentence: La "navette" qui DEVAIT nous amener à l'hôtel ÉTAIT en fait un vieux bus de campagne cabossé dans lequel on A PU apprécier toutes les aspérités de la route.- There is a nuance of meaning between devait and a dû that kind of reflects the nuance of length: L'imparfait de devoir would be more like "was supposed to", whereas its Passé Composé is usually more like "had to/must have". I hope that distinction can help you a bit there :)- the choice of était is quite clear here: it's a description of the ongoing state of the hotel at the time, hence L'Imparfait.- as for the difference between pouvait and a pu here, here it's about your point of view. If you were telling this anecdote from the point of view of  still being on the bus and therefore considering the action in its ongoing development (i.e. we were able to feel that as the bus was driving), you would use L'Imparfait.However, here you telling the story from a later point of view, as a sum up on that bus journey you're no longer on (we felt it while we were on it, which is not the case any more), hence Le Passé Composé.I hope that's helpful!À bientôt !