Is it just me or is this one very ambigous? Recuerdan could mean you or they remember, and su cara can be his/her/your/(their?) face.
With ustedes, it can only mean you. Without the ustedes, it could be either you (plural) or they.
It is ambiguous, yep. "Their" probably doesn't work, since "cara" is singular, but "su" could even refer to "its" (the face of a watch).
I would argue that "their" works well here actually, because (at least in my experience) when referring to something that belongs to someone else (like a face) when you don't know the gender of that person, many people will use "their" instead of "his" or "her" even when referring to just one person.
Can this also translate to "do you remember their faces?" and "do you remember your faces?" (let's say in a hypothetical situation where a group of people vowed never to look in a mirror).
I remember several lessons ago someone pointed out that when referring to multiple people's faces (or other singular body parts), you'd still use the singular form for the object, since each person only has one face.
No, that wouldn't really work here. It (likely, there might be another mechanism where that idea works, too) has to do something with whether you're using a reflexive/objective form of the verb or not. If you use the objective form (something like "Le miré la cara" - "I looked him in the face"), the pronoun already specifies which individual you're considering, so you'll only talk about the body parts of that person. Meaning, if you use the se/te/le/etc. form of the verb, you'll only talk about one face, one mouth, one body, two hands, two ears, and so on, anything that is part of one single person. If you don't use that objective form, you're talking about the collective body parts of all people you include.
- Les besó la boca. - She kissed them on the mouth.
- Besó sus bocas. - She kissed their mouths.
The objective form doesn't function like that with recordar (presumably because you don't actually do something to those people), so in this sentence you have to use the plural "sus caras".
Your sentence is in past tense. That would be "¿Recordaron (ustedes) su cara?"
To me this would translate as, "Do you all remember his face?" As if the speaker were addressing a group of people.
Yes, but it is not necessary to put the "all" in the sentence. "You" is plural without the "all."
Ya'll, though it is accepted and understood in many regions of the United States, may not be easily understood as "you all." I live in the US in the midwest and I would grasp it quite quickly, however another region may never have used "ya'll" in spoken English. Gracias!
I agree, but it is extremely informal. I'd love to see Duo accept it, but I'm not surprised that they don't.