More on Have To/Must

I know I've already been over the grammar points for expressing "have to" or "must", but Tobira has added a few details that are worth mentioning. First, both of the following two forms can be used interchangeably:

～なければ　いけない / ならない
～なくては　いけない / ならない

The first is shortened to なきゃ while the second is shortened to なくちゃ. I know I posted those forms earlier, but I didn't realize where each was coming from, so that makes more sense now. Also, ならない can be shortened to なんない.

Next, although いけない and ならない can both be used, ならない is "commonly used when the speaker states his/her sense of obligation, while いけない is commonly used when the sense of obligation is directed toward the hearer (i.e., when the speaker wants to impose a certain obligation on the hearer)." Thanks, Tobira.

国民はみんな税金を払わなければなりません。
All citizens must pay taxes.

このクラスの学生は十二時半に食堂に行かなければいけないんだ。
Students of this class have to go to the cafeteria at 12:30.

今朝、朝寝坊してクラスに遅れちゃったから、明日はもっと早く起きなくちゃ。
I overslept this morning and was late for class, so tomorrow I should get up earlier.

リーさんはケーキが欲しいと言ってたから、店に行って買わなくちゃ。
Lee-san said he wants a cake, so I have to go to the store and buy it.