If you quote the full sentence here, I may be able to shed some light on your question.

Thanks. I thought this question would appear under the solution text, but not so. (Also, FAO staff, I'm still not getting email notifications of replies, just of "likes", which is how I came back to see this reply...)

This is a good one. ;)

J'ai toujours détesté... -- The passé composé here means that the speaker still loathes the commute today. Using the imperfect tense (je toujours détestais...) would mean that he was hating it in the past but not in the present.

...que je devais faire... -- The imperfect here is the standard flavor of imperfect: a continuous action without reference to beginning and ending. A kind of background action.

I know what you're thinking now: isn't the loathing (a détesté) also a kind of background action that goes hand in hand with the having to do the commute (devais faire...)? And why not use the same tense for both?

As it reads now, the subtext is that the loathing is continuing into the present while the commuting seems to be a thing of the past. Even though this might not be factually correct, that's the emphasis the speaker puts on it.

Well if it's a rhetorical nuance rather than a rule of grammar, I'm less unhappy I missed it. :-) Thanks for that very full and comprehensible answer. Would it be ok to say then that the imperfect in the second verb is optional, and "j'ai dû faire" would be at least permissible, given the commute is ongoing?

Adrian

For me, there is a problem in that the original script is in English, but in my view the tenses used in the original script are not the tenses that would be used in English, given the fact that the issues discussed are current/ongoing at the time, and will not stop for another month. 

It seems to have been written to try to preview the tense changes needed from English to French - may be reasonable for the B1 level to some extent, but ultimately confusing, I think.

I think the intent of the article to draw on use of imparfait for the most part of the exercise would be better covered if the last sentence was something like : 

" But today, I am moving (have moved) to an apartment that is much closer to my office, and I'll be able to go (there) on foot or by bike. It's going to change my life! " 

The preceding sentences then would not need to change.

It would remain passé composé for " j'ai toujours détesté .. " because that statement from the past is (would be)  still true ( impactful as discussed on Quora ) in the present. He still detests (it), but no longer will have to do 'it'.  This use of passé composé apparently derives from its original usage when passé simple was also in routine use in (spoken) French. 

Link to excellent YouTube discussion on imparfait and passé composé by Hugo Coton - the pertinent section starts at about 13:40, but the whole video is worth listening to ( in comprehensible French)

https://www.youtube.com/watch?v=3rpQ5xeFneg 

An interesting Quora discussion of passé simple and passé composé, and their differences, as well as the continued use of passé simple in French literature  

https://www.quora.com/What-is-the-use-of-the-pass%C3%A9-simple-in-French-grammar#

I mostly agree with Maarten, but I don't think "j'ai toujours détesté" implies that it is still true in the present. That would require "I have always hated" in the English text, which sounds odd to me - why would you continue to hate something that has stopped?

Instead, I think this is the other function of the passé composé, a completed action in the past. Maybe the reason that this is not in the imparfait is that it's seen as a single action, rather than a repeated action like the commuting. I don't think it would be wrong to use the imparfait, however, where it would have the sense of "always used to hate", which would still be consistent with the English.

Since Cécile mentions my comment, let me clarify that I think "I always hated" can mean exactly the same as "I always used to hate". So I think that "I always hated" can be correctly translated by either the imparfait or the passé composé.