"I am sending you a letter."
Translation:Wysyłam ci list.
"You" could indeed refer to singular or plural, but if you want to interpret it as singular, then it would be "Wysyłam Tobie list".
"ty" is used for subjects, but it's an indirect object here.
Could "ci" be used in place of "wam/tobie"?
Isnt wysyłam past tense? I thought all verbs with -łem/-łam endings where past tense
Well, I guess only 'almost all', then...
"Wysłałem/Wysłałam" is past tense.
Wrote my instinct answer which was "Ci wysyłem list" and got it wrong. Is this where the "emphatic pronoun tobie" comes in (TOBIE wysyłem list a nie jemu - meaning "I sent YOU the letter and not him")?
Yes, if something, than "tobie" should be used here. But the construction with "tobie" at the beginning wouldn't feel very natural anyway, it feels to me like you're correcting someone who didn't hear well what you just said.
As almost any sentence about "you", this has at least two equally correct answers. "Wysyłam wam list" (plural) is one, and "Wysyłam tobie list" (singular) is the second one. I actually think that "tobie" is a bit less natural than "ci" in this sentence so I made "ci" a starred answer instead of "tobie". Anyway, if "wam" was not on the list, then the singular choice must have been.