Should vs. Could vs. Need vs. Must: Explained in English

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 a soft expectation: You should save your work.
• Could = a real possibility, an option, or a polite request: We could leave now; Could you help me?

Strength from strong to soft

Think about how strong you want to sound.

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

• Choose must for rules, laws, or strong logic: That must be the right door.
• Choose need (to) for required steps: You need to sign in first.
• Choose should for friendly advice: You should take a break.
• Choose could for choices and ideas: You could try a different route.
  This ladder helps you match the verb to your purpose.

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.

For polite questions, could is softer than can: Could you email the file? (more polite) vs. Can you email the file? (neutral).

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 that did not happen: You should have saved the draft.
• Use must have for a strong guess about the past: She must have left already.
• Use could have for a possible past event that you are not sure about: It could have rained last night.

Fast choices (B1+ checklist)

Ask yourself three things: Is it a rule? Is it necessary? Is it only advice or a choice?

• Rule or strong logic? Use must.
• Real requirement but not a rule? Use need (to).
• Advice or soft expectation? Use should.
• Option, possibility, or polite request? Use could.
  For US readers, prefer must not, don't need to, and don't have to. Avoid mustn't and needn't in everyday writing.

Usage examples (original, 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.
• [B2] Should have (past advice): You should have backed up the database before the update.

FAQs

Q1: Is “must” stronger than “have to”?
Yes. Must sounds more official and is common in rules. Have to is everyday obligation. In speech, Americans often use have to; in written rules, must is preferred.

Q2: What is the difference between “must not” and “don't have to”?

• Must not = not allowed.
• Don't have to / don't need to = optional.

They do not mean the same thing.

Q3: Can I use “needn't” in the US?
People understand it, but it sounds formal or British.
In US English, use don't / doesn't need to.

Q4: Which is more polite for requests, “can” or “could”?
Could is more polite and softer than can: Could you open the window, please?

Q5: How do I talk about necessity in the past?
Use had to: We had to cancel the meeting. For past advice that did not happen, use should have: You should have called earlier.