"I am sending you a letter."
Translation:Wysyłam ci list.
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.