There has been some discussion of the new sentences like this in the course. It is my opinion that they have been added in error and it is now my recommendation to the team that they not be included in a future tree. A few other fluent speakers (including members of the akademio) have suggested similar alternatives to what I'm about to suggest below.
A better sentence would be: - Ili devus esti pagintaj
The sentence breaks down as follows: - They obligated-not-fulfilled to-be paid-in-the-past. - There is obligation for them which they did not fulfill for them to have paid. - They should have paid.
Devus is a special case and can have two meanings - hypothetical obligation ("would have to"), or obligation which hasn't been met. Also note that all expressions with -us are timeless and don't indicate past, present, or future. They only indicate contrary-to-fact, hypothetical.
The problem with ili estus devintaj pagi is that it breaks down as follows. - They would be in the state of having been obligated to pay. - They would have had to pay.
It doesn't specify that they actually were in that sate. It also doesn't say whether they actually paid.
Devintus contains all the problems with "estus devintaj" and compounds it by treating the verb ending as a form of estas, which, as has been explicitly stated since the early years of Esperanto, it is not. My sense is that the people who use it are carrying over national language habits and they don't really understand how the -us ending words. I recommend not using it.
- Ili devus pagi = they should have paid / they would have to pay
- Ili devus esti pagintaj = they should have paid / they would have to have paid.
Your post, explanations, and examples are very helpful, as usual, Salivanto. I do have a few questions/observations, however. Regarding the use of verbal endings on adjectives to substitute for esti, I see this a lot. For instance: "Gxi bluas" to mean "It's blue" as opposed to "gxi bluigas" (it's making something blue") or "gxi bluigxas" (it's turning blue).
So, it does seem there is some precedent for verbal endings substituting as "esti" on adjectives. Also, when I first learned Esperanto, I was also taught that one could say things like: "Mi skribantis" as a substitute for "Mi estis skribanta." What's your take on this? Is it just doing this with "estus" or the "-us" ending that is problematic?
Devus is a special case - as I said above - and is distinct from "La birdo bluas".
"My take" on that form (bluas) is best described in my blog post on that topic.
As for "Mi skribantis" - I think I just threw up a little in my mouth.