Must is a modal verb in English that conveys a sense of necessity, obligation, or certainty. It has various applications in different contexts, which are detailed below:

Basic Meanings of Must

1. Necessity or Obligation:

   • Must primarily means "have to" or "be required to" do something. It expresses a strong sense of necessity or obligation.
   • Example: You must study hard to pass the exam. (你必须努力学习才能通过考试。)

2. Certainty or Logical Inference:

   • Must can also be used to express a high degree of certainty or a logical inference based on available evidence.
   • Example: This must be the right way. (这一定是正确的路。)

Applications in Different Contexts

1. In Statements:

   • When used in a statement, must conveys a direct and emphatic sense of necessity or obligation.
   • Example: You must complete your homework before going out to play. (你必须先完成作业才能出去玩。)

2. In Questions:

   • In questions, must can be used to seek confirmation or express surprise about a certain fact or situation.
   • Example: Must I finish this report today? (我今天必须完成这份报告吗？)
   • Note: When answering a must-question negatively, we usually use needn't or don't have to instead of mustn't.

3. In Negatives:

   • Mustn't means "do not have to" or "are not allowed to" in a negative sentence, expressing a strong prohibition.
   • Example: You mustn't play with fire. (你不准玩火。)

4. Expressing Inference:

   • Must can be used to make inferences about the present, past, or future, although it is more commonly used for the present and past.
   • For the present: He must be at home now. (他现在一定在家。)
   • For the past: She must have finished her work by now. (她到现在一定已经完成了工作。)

5. In Passive Voice and Progressive Forms:

   • Must can be followed by passive structures or progressive forms to express necessary actions or ongoing situations.
   • Example: The work must be done by Friday. (这项工作必须在星期五之前完成。) / He must be studying for the exam right now. (他现在一定在准备考试。)

Emotional Tone and Usage Tips

• Must carries a strong emotional tone, often conveying a sense of urgency or importance.
• It is important to use must appropriately in different contexts, avoiding overuse or misuse that may cause misunderstandings or offense.
• When expressing suggestions or possibilities, other modal verbs such as should, could, or may may be more appropriate choices to avoid sounding too demanding or certain.

In summary, must is a versatile modal verb in English that can express necessity, obligation, certainty, and inference in various contexts. Understanding its different meanings and applications will help you use it more accurately and effectively in communication.