Overview
| Modal | Meaning | Example |
|---|---|---|
| must | Strong personal obligation (you decide) | I must call my mum today. |
| have to | External obligation (a rule or someone else decides) | You have to wear a uniform at school. |
| should / ought to | Advice or weaker obligation | Everyone should wear a seatbelt when driving. |
| mustn't | Prohibition — it is forbidden | You mustn't smoke inside the school. |
| don't have to / needn't | Lack of obligation — it is not necessary | You don't have to pay — the museum is free. |
Obligation: must vs have to
| must | have to |
|---|---|
| Personal / internal obligation — you feel it's necessary | External obligation — rules, laws, authority |
| I must study more. (I think so) | I have to be at work by 8:00. (my boss says so) |
| You must try this cake — it's amazing! | You have to show your passport at the border. |
Past obligation:
Only had to exists in the past — there is no "musted": ✓ "I had to work on Saturday." ✗ "I musted work."
The key contrast: mustn't vs don't have to
| mustn't | don't have to |
|---|---|
| Prohibition — forbidden, do NOT do it | No obligation — not necessary, but you can if you want |
| You mustn't be late. (Don't be late!) | You don't have to rush. (No hurry.) |
| Children mustn't tell lies. | She doesn't have to do it today — she can do it tomorrow. |
| You mustn't forget to lock the doors. | They don't have to get up early — it's Sunday. |
You mustn't drink the water — it's contaminated! (= forbidden)
You don't have to drink the water if you don't want to. (= your choice)
You don't have to drink the water if you don't want to. (= your choice)
Exercises
mustn't or don't have to?
1. We have a lot of work tomorrow. You ___ be late.
2. The museum is free. You ___ pay to get in.
3. You ___ smoke inside the school.
4. We ___ rush. We've got plenty of time.
5. We ___ forget to lock all the doors before we leave.
6. They ___ get up early today, because it's Sunday.