have to, must

Using “have to,” “must,” “mustn’t,” and “don’t/doesn’t have to” in English

1. Obligation: “have to” and “must”

Meaning: Both express that something is necessary.

Form:

• have to: subject + have/has to + base verb
  Example: “She has to leave early.” (It is necessary for her to leave early.)
• must: subject + must + base verb
  Example: “You must wear a seat belt.” (It is required; very strong obligation.)

Use: “Must” often sounds stronger, more urgent, or more formal. “Have to” is more common in everyday speech and often refers to external rules.

2. Prohibition: “mustn’t”

Meaning: Something is not allowed; it is strictly forbidden.

Form: subject + mustn’t + base verb

Example: “You mustn’t touch the wires.” (It is forbidden to touch them.)

3. Lack of obligation: “don’t/doesn’t have to”

Meaning: Something is not necessary; you can do it, but you don’t need to.

Form: subject + don’t/doesn’t have to + base verb

Example: “He doesn’t have to work today.” (It is not necessary for him to work.)

4. Important contrasts

• don’t/doesn’t have to ≠ mustn’t
  “Don’t/doesn’t have to” = not necessary.
  “Mustn’t” = not allowed.

Examples:

• “You don’t have to come if you’re busy.” (You can choose.)
• “You mustn’t come into the lab without goggles.” (It is forbidden.)