In Japanese, there are many ways to say “you have to ~,” like "〜なければならない," "〜なくてはいけない," "〜ないといけない," "〜ないとだめ," and "〜なきゃ." They all mean "you must do ~." For example: "行かなければならない," "行かなくてはいけない," "行かないとだめ," and "行かなきゃ"—they all mean the same thing. When you want to speak politely, you use forms like "〜なければなりません" or "〜なくてはいけません." But when speaking with friends, you often say "〜ないと" or "〜なきゃ." It was hard at first because the word becomes lengthy and difficult to read, but now I think it’s interesting because even if the forms are different, the meaning is the same. I'm not sure, is there a reason why there are several forms for "you have to" aside from polite and casual?