What These Words Mean

Should, could, need (to), and must are modal verbs. They help you show advice, possibility, necessity, and strong rules.

• Must = a rule or something you are sure about: You must wear a badge.
• Need / need to = necessary for a task: You need to charge your phone.
• Should = good advice or soft expectation: You should save your work.
• Could = a real possibility or polite request: We could leave now; Could you help me?

Strength from Strong to Soft

Use this ladder: must → need (to) → should → could

• Choose must for rules, laws, or strong logic
• Choose need (to) for required steps
• Choose should for friendly advice
• Choose could for choices and ideas

Negatives and Questions (US Style)

In US English, say must not (not mustn't) to mean not allowed: You must not share the code.

To say not necessary, use don't/doesn't need to or don't/doesn't have to: You don't need to come today.

Be careful! must not ≠ don't have to:

• Must not forbids
• Don't have to makes it optional

Talking About the Past

• Use had to for past necessity: We had to leave early.
• Use should have + past participle for advice in the past: You should have saved the draft.
• Use must have for a strong guess: She must have left already.
• Use could have for a possible past event: It could have rained last night.

Usage Examples (CEFR-tagged)

• [A2] Should (advice): You should bring water; it's a long walk.
• [A2] Need to (necessity): I need to finish this form before 5 PM.
• [B1] Must (rule): All visitors must sign in at the front desk.
• [B1] Must not (not allowed): You must not use your phone during the test.
• [B2] Could (option): We could book an earlier flight if the price drops.

FAQs

Q: Is "must" stronger than "have to"? A: Yes. Must sounds more official. Have to is everyday obligation.

Q: What is the difference between "must not" and "don't have to"? A: Must not = not allowed. Don't have to = optional.

Q: Which is more polite for requests, "can" or "could"? A: Could is more polite and softer than can.