"See you on Monday."
Translation:Los veo a ustedes el lunes.
That's just how it is in Spanish really. To say "on Monday" you literally say "the Monday".
I think it's one of those special things about the language. Sort of like how English often omits the word that (as in "the shirt (that) you bought"), whereas you have to have "que" in the same phrase in Spanish.
In my experience, when Duo doesn't specify singular or plural, I've found it accepts either. I try to switch it up to keep myself from getting too lazy.
If it marked you wrong for this, that would certainly be odd. However, I'd make very certain that you weren't marked wrong for mixing singular and plural, or some other error. You have to be consistent with your sentence structure, depending on which you choose (singular or plural).
I put "Te miro el lunes" and it wouldn't accept it until I changed it to "A tí te miro el lunes"
I understand what it means to put it at the front but I was taught that it was unnecessary since in this instance it adds no information. Is it more common to use or not use this in vernacular Spanish?