I wrote "tomorrow i will send a letter," which was marked as wrong. However, there is no "you" specified in the sentence - yes, it is implied, but not definitely there as the translation suggests.
As I understand it, 주 in 보내줄 means the sending is for someone else's benefit. ..to or for someone else. To just 'send' would be 보내다 compared to 보내주다.
couldn't this be the imperative form as well? like, telling someone to send a letter tomorrow?
Im not fluent, but since there is no subject and its informal, i believe so
Nope, imperative is only used with present tense forms. It's true even in English, e.g. "send this letter tomorrow"
No subject but only accepts "I" for subject
솔직히 듀오링고 너무 문법에만 시각을 맞추는 것 같다...
來日 便紙를 보내줄 거야!
Please accept more conjugations. 거야 seems the only acceptable answer right now.