What the problem did you meet when someone(English learner) talked with you in English??

How to use this word "must"? anser in English or Chinese please tell me this how to use this word in any situation.I alway forget its construction = =||| had better have sentence example ps:(Does the preposition(in) used correctly in this sentence.or it should use on、at?)

Have you ever meet the problem about internet connection on italki? connection always broke when I used the function of "Chat Now" and then web page closed down automatic why?