must 必须做某事
must't 绝对不能做某事
have to 不得不做某事，必须做某事
don't have to 不是必须做某事，可做可不做，可以选择
I must study English so ...
must .... because...
have to ...so ...
have to ...because...
must't ....so..
must't ....because...
don't have to...so...
don't have to ...because...