Actually, this is the first time I have seen this construction introduced. I got it right just by thinking "that sounds like 'có' " but I wouldn't have guessed the meaning.
Sigh, I wish Duolingo would drop these "gotcha!!" exercises. Explain the material to be covered, then give exercises covering it. Don't introduce new material in the middle of the testing out of the blue.
How in the hell do you 'just learn it' when you get no practice on it?? I had already completed all five levels of this module and never saw this sentence and now it apparently has been added. I have only seen it twice now (once via this comment email). To make things worse, DL now disincentives 'deep dives' into completed lessons to catch this.
So how in the hell is someone suppose to 'just learn it' when you only see it once in a blue moon? If you really want someone to learn something efficiently, you give them repeated experiences within a structured setting. Haphazard exposures don't cut it.
I have Vietnamese friends who help (but believe me, when one of them sat down with me in Vietnam and advised me what to write down for an answer while I was doing a pass through a module, DL marked his input wrong!!) and I have compiled everything I have covered in a big spreadsheet. I too have looked into other resources online and have found some, but have stayed here despite the frustration and pain b/c I am stubborn enough to want to get through the whole course (all five crown levels) and reasonably know the material (to get as much of it into active memory as possible) before I move on to something else. That, plus I don't want to diffuse my efforts.
The DL Vietnamese course's biggest problem is that the material past the first third or so of the course is not taught, you're just thrown to the linguistic wolves. So you end up self-teaching, and when you self-teach you make a lot of wrong assumptions and correlations based on your limited experience, or what you've been told by others.