Obligation, Necessity, and Permission

TL;DR

This note explains how to express what you must do (obligation/necessity) and what you can do (permission) in English. We'll cover common modal verbs like must, have to, should, can, and may, and when to use each appropriately. Understanding these helps you communicate rules, advice, and possibilities clearly.

1. The Mental Model

Think of these concepts as different levels of "push" or "pull" on an action. Obligation and necessity are strong pushes, permission is a green light, and advice is a gentle suggestion.

2. The Core Material

When talking about what someone has to do, needs to do, or is allowed to do, English uses several verbs, mainly modal verbs. The choice of verb often depends on the strength of the obligation, the source of the necessity, or the formality of the permission.

Obligation and Necessity

Photo by Pixabay on Pexels

These refer to things that are required or important to do.

• Must: Used for strong, internal obligations (you feel it's necessary) or strong recommendations, often from the speaker. It also expresses certainty or logical deduction.

  • Example (internal obligation): "I must finish this report by Friday." (It's important to me.)
  • Example (strong recommendation): "You must see that new movie!"
  • Example (certainty): "She hasn't eaten all day; she must be hungry."
  • Mustn't (must not): Expresses a strong prohibition – something you are not allowed to do.
    • Example: "You mustn't use your phone during the exam."

• Have to: Used for external obligations, rules, or laws. The necessity comes from outside the speaker. It's often more common in everyday speech than "must" for general obligations.

  • Example (external rule): "We have to wear a uniform at school." (The school requires it.)
  • Example (necessity): "I have to wake up early tomorrow for my flight."
  • Don't have to: Expresses a lack of obligation or necessity – you are free not to do something, but you can if you want. It's not a prohibition.
    • Example: "You don't have to come if you're busy." (It's not required.)

• Should / Ought to: Used for advice, recommendations, or what is generally considered a good idea. It's a weaker form of obligation than "must" or "have to." "Ought to" is more formal and less common.

  • Example: "You should study more for the test." (It's good advice.)
  • Example: "We ought to call her and apologize."

Permission

These refer to things you are allowed or authorized to do.

• Can: The most common and versatile way to express permission. It's informal. Also used for ability.

  • Example (permission): "You can leave early today."
  • Example (ability): "I can speak three languages."
  • Can't (cannot): Expresses lack of permission or inability.
    • Example: "You can't park here." (It's not allowed.)

• May: A more formal way to ask for or give permission. It can also express possibility.

  • Example (asking permission): "May I come in?" (More polite than "Can I come in?")
  • Example (giving permission): "You may begin the exam now."
  • Example (possibility): "It may rain tomorrow."

• Could: Used to ask for permission more politely than "can." It's also the past tense of "can" for ability.

  • Example (asking politely): "Could I borrow your pen for a moment?"

Here's a quick flow to help you decide which to use:

graph TD
    A["Is it a rule or a strong internal necessity?"] --> B{Source of Obligation?};
    B -- "External rule/law" --> C["Use: Have to"];
    B -- "Internal feeling/speaker's strong opinion" --> D["Use: Must"];
    A -- "Is it advice/recommendation?" --> E["Use: Should / Ought to"];
    A -- "Is it about prohibition?" --> F["Use: Mustn't"];
    A -- "Is it about no obligation?" --> G["Use: Don't have to"];
    H["Is it about permission?"] --> I{Formality?};
    I -- "Informal/General" --> J["Use: Can"];
    I -- "Formal/Polite" --> K["Use: May / Could"];
    H -- "Is it about no permission?" --> L["Use: Can't"];

3. Worked Example

Let's imagine you're planning a trip with a friend. Here's how you might use these concepts:

"Hey, I've been looking at flights. We have to buy tickets soon because prices are going up. We should also book a hotel in advance, or we might not find a good place. Remember, you mustn't forget your passport! I've packed my swimwear, but you don't have to bring yours if you're not planning to swim. Also, if we finish sightseeing early tomorrow, we can go to that market we talked about. May I suggest we try the local food for dinner tonight?"

• "have to buy tickets" (external necessity - prices are rising)
• "should also book a hotel" (advice/recommendation)
• "mustn't forget your passport" (strong prohibition/absolute necessity)
• "don't have to bring yours" (lack of obligation/choice)
• "can go to that market" (permission/possibility - if we finish early)
• "May I suggest..." (polite request/suggestion)

4. Key Takeaways

• Must expresses strong, internal obligation or prohibition, often from the speaker.
• Have to expresses external rules, laws, or necessities.
• Should gives advice or recommendations, suggesting what is good to do.
• Don't have to means something is not obligatory, you have a choice.
• Mustn't means something is strictly forbidden.
• Can expresses informal permission or ability.
• May is for formal permission or possibility.

Common Mistakes to Avoid:
- Using "mustn't" when you mean "don't have to." "You mustn't pay" means it's forbidden to pay, while "You don't have to pay" means payment isn't required.
- Confusing "can" and "may" for permission; "may" is generally more formal and polite.
- Forgetting that "have to" changes form with different subjects (e.g., "he has to," "they have to").

5. Now Try It

Imagine you are writing a list of rules for your new roommate. Write five sentences using different verbs we covered (must, have to, should, can, mustn't, don't have to) to explain what they need to do, what they're allowed to do, and what they absolutely shouldn't do. For example, "You must keep your room tidy." Success looks like using each verb type correctly and appropriately for the context of roommate rules.