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?