랑 is used like 와 and 하고.
얘기하다 is an abbreviation for 이야기하다.
랑 is used in speech specifically though and since this part of thr course is casual, that's probably why they used it. 와 and 하고 are used in writing. They should have clarified why the used 랑 here though.
서연과 이야기하자 > is not accepted but in another exercice 선생님과 이야기하자 is accepted... why is it wrong here? @duolingo: thanks in advance for the grammatical explanation
My guess is that talking about 선생님 requires honorific no matter which speech level you use.
서연이랑 이야기해 not accepted. Surely ~하자 and ~해 should both be acceptable for casual speech?
서연이랑 이야기해 is Talk to 서연 (it is like order), where as 서연이랑 이야기하자 is Let's talk to 서연 (it is suggestion).
Could we also say "서연이랑 말하자"?