On desktop, when you finish with a sentence you will either get the green box for a correct answer or the red box for an incorrect answer. There are two options in the bottom left of that, "Discuss Sentence" and "Report a Problem". Click the "Report a Problem", to give feedback.
I think so. In Portuguese (My native language), for example, we greet people saying "Oi/Olá, tudo bem?", wich literally translates to "Hi, how are you?". You're not supposed to answer the question though: It is only there to keep conversation going, kinda serving like a bridge to connect the "Hi/Hello" to the subject of the conversation itself. I image it works like that in Swahili too, but, only as a beginner, I can't be sure.