Must, Have To, and Should: Expressing Obligation and Advice
EnglishMam Curriculum Team
Verified SEO ESL Expert
Modal verbs can change the strength of a sentence. Let’s learn how to express rules, duties, and advice.
1. Obligation & Rules (Have To vs. Must)
Use have to for external obligations (laws, company policies, rules). Use must for strong personal obligations (something you feel is essential).
- “We have to submit the tax returns by Friday.” (Tax law requirement).
- “I must call my mother tonight.” (My personal feeling of duty).
2. Giving Advice (Should)
Use should to give suggestions, advice, or friendly recommendations. It is not an obligation.
- “You should prepare the slides before the client call.” (Good advice).
3. Prohibitions vs. Lack of Obligation (Mustn’t vs. Don’t Have To)
This is a major source of confusion. The negative forms have completely different meanings:
| Modal Verb | Meaning | Example |
|---|---|---|
| Mustnot / Mustn’t | Prohibited. It is against the law/rules. | “You mustn’t share password details.” |
| Don’t have to | Optional. There is no obligation, but you can if you want. | “You don’t have to wear a tie in this office.” |
Incorrect
“You mustn’t pay for the water, it is free.” (Implies it is illegal to pay).
Correct
“You don’t have to pay for the water, it is free.” (Implies payment is optional).
End-of-Lesson Checkpoint
Test your understanding of the concepts taught in this lesson. Answer the questions below and get instant explanations.
1. I ___ a bad cold since last Wednesday, but I am finally feeling better today.
Explanation: The sentence uses "since last Wednesday", indicating a duration that started in the past and continues into the present (unfinished timeframe). This requires the **Present Perfect**.
Explanation: "Had" (Past Simple) is used for finished time. "Since last Wednesday" connects the past to the present, requiring the **Present Perfect** ("have had").
2. We ___ to London in 2021 for a business language seminar.
Explanation: The timeframe "in 2021" is completely finished. Therefore, we must use the **Past Simple** ("went").
Explanation: "In 2021" is a finished, specific past time, which is a classic trigger for the **Past Simple** ("went"). "Have gone" would mean the timeframe is unfinished.
3. ___ you ever ___ a client presentation in English before?
Explanation: We are asking about life experience ("ever") up to the present moment without a specific finished time. This requires the **Present Perfect** structure ("Have" + past participle "delivered").
Explanation: For general life experience questions ("ever"), use the **Present Perfect** ("Have you delivered"). Past simple ("Did you deliver") would require a specific past time trigger.