Dic: must (as a modal of intention or resolution, & of emphasizing your point or conviction)

■■ intention or resolution

  ■ You use must to express your intention to do something.

• I must be getting back...
• I must telephone my parents...
• He told the Prime Minister that he felt he must now leave.

  ■ used as an auxiliary to express resolution

1. on the part of the speaker when used with I or we: I must finish this.
2. on the part of another or others as imputed to them by the speaker, when used with you, he, she, they, etc.: let him get drunk if he must.

■■ emphasizing your point or conviction

  ■ You use must in conversation in expressions such as `I must say' and `I must admit' in order to emphasize a point that you are making.

• This came as a surprise, I must say...
• I must admit I like looking feminine...

  ■ (used emphatically) used as an auxiliary to express conviction or certainty on the part of the speaker

• he must have reached the town by now, surely.
• you must be joking

.... COBUILD, COLLINS