without specifying?

Question

Ils devaient venir ce soir, mais ils ont annulé à la dernière minute.They were supposed to come tonight, but they cancelled at the last minute.

When conjugated in L'Imparfait (Indicatif), devoir refers to a past obligation, without specifying whether it was met or not.
Actually, in most cases, the obligation was not met.
The first example in the above lesson definately specifies that they didn't come when supposed to. How is that complying with this rule "without specifying whether it was met or not"

Accepted Answer

Paul,
‘ Devoir faire qqc ‘ in imparfait is similar to the use of ‘ (was/were) supposed to do something ‘ in English. 
That is the case in this sentence - it was not ‘compulsory’ or an ‘obligation’, but something they were planning/going to do. 
 
The example fits the lesson discussion well, which is easier to see if you break the sentence down : 
 ‘ Ils devaient venir ce soir ‘ can stand alone as a sentence, and would not confirm whether they came or not, although without further context it would raise suspicion that they didn’t. 
The reason we know definitively that the visit didn’t happen is not because devoir was in the imparfait, but because of the context provided by the additional clause in the sentence “ mais ils ont annulé à la dernière minute. “  
This is the same as in English when we say something was ‘supposed’ to happen - it usually means it didn’t, but this is only known for sure when further information is provided. 
Occasionally, ‘devoir in imparfait’ in French, or ‘supposed to ‘ in English, can be followed by a “surprise” explanation that ‘ it ‘ actually did happen, but that is not the norm in either language !