Life is full of rules, duties, and advice, and English uses different verbs for each level of pressure. 'You must wear a helmet' is a strong rule. 'I have to work on Saturday' is a necessity from outside. 'You should see a doctor' is friendly advice. If you mix these up, you can sound bossy when you only want to suggest, or too soft when something is really required.

You also need these verbs to understand signs, contracts, emails from your boss, and safety instructions. 'Passengers must fasten their seatbelts' is not a suggestion. Reading and using must, have to, need to, and should correctly keeps you safe, polite, and clear.

First, learn the forms, because they follow two different patterns. Must and should are modals, so they take the bare base verb: 'You must stop', 'You should rest'. Have to and need to are normal verb phrases, so they keep 'to' and they change with the subject and tense: 'She has to work', 'I had to wait', 'Does he need to sign?' Mixing the two patterns creates errors like 'must to sign', which is never correct.

Now the meanings. Must and have to both express strong obligation: the action is necessary, not optional. Must often sounds like the speaker's own rule or a written rule: 'You must switch off your phone in the exam.' Have to often points to outside pressure such as a job, a law, or a timetable: 'I have to catch the 7 a.m. train.' In everyday talk, have to is more common; must can sound formal or very strong.

Need to is close to have to but focuses on necessity: something is needed for a result. 'You need to book early if you want a window seat.' Should is much weaker. It gives advice or an opinion about the right thing to do: 'You should drink more water.' Nobody is forced; it is a recommendation.

The negatives are a trap, because must and have to split apart. Mustn't means it is forbidden: 'You mustn't smoke here' means do not do it. Don't have to and don't need to mean the opposite: no obligation, it is your choice. 'You don't have to come to the meeting' means you can come if you want, but it is not required. Shouldn't gives negative advice: 'You shouldn't eat so late.'

For past and future, must has no forms, so borrow from have to: 'I had to renew my visa last month', 'You will have to show your passport'. So think of a pressure scale: must and have to at the top (required), need to just below (necessary), should at the bottom (a good idea).

1. 1. All visitors must report to reception first.

   A written rule with 'must' plus the base verb 'report'.

2. 2. I have to finish this report by five o'clock.

   Outside pressure from work uses 'have to' with 'to'.

3. 3. She has to take two buses to get here.

   'Have to' changes to 'has to' with she; modals never change like this.

4. 4. You should try the soup; it's really good.

   'Should' gives friendly advice, not an order.

5. 5. Do we need to bring our own towels?

   'Need to' makes questions with 'do', like a normal verb.

6. 6. You mustn't touch that wire; it's dangerous.

   'Mustn't' means the action is forbidden.

7. 7. You don't have to pay now; you can pay later.

   'Don't have to' means no obligation, which is very different from 'mustn't'.

8. 8. I had to cancel my trip because of the storm.

   Past obligation uses 'had to', because 'must' has no past form.

9. 9. You'll have to change trains in Berlin.

   Future obligation uses 'will have to'.

10. 10. He shouldn't drive when he's this tired.

    'Shouldn't' advises against an action.

11. 11. We need to buy milk before the shop closes.

    'Need to' shows something is necessary for a result.

12. 12. Should I email the client today or tomorrow?

    Question with a modal: 'should' moves before the subject, no 'do'.

13. 13. Patients must not eat before the blood test.

    Formal written prohibition with 'must not'.

14. 14. You don't need to print the ticket; your phone is fine.

    'Don't need to' removes the necessity; the choice is yours.

The most common mistake is adding 'to' after must or should: 'You must to sign in', 'You should to rest'. This happens because have to and need to do use 'to', so the patterns get mixed. Keep them apart: modals take the bare verb, normal verbs keep 'to'. Wrong: 'You must to sign in.' Right: 'You must sign in.' And the mirror mistake also happens: 'I have go now' drops the needed 'to'. Right: 'I have to go now.'

The second mistake is treating mustn't and don't have to as the same. They are almost opposites. 'You mustn't tell anyone' means it is forbidden; keep the secret. 'You don't have to tell anyone' means telling is not necessary, but you may if you like. Choosing the wrong one can turn a free choice into a ban, so stop and think: forbidden, or just not required?

A third mistake is using must for the past: 'I must work late yesterday.' Must has no past form for obligation, so English borrows had to. Wrong: 'I must work late yesterday.' Right: 'I had to work late yesterday.' The same trick works for the future: 'You will have to wait', not 'You will must wait.'

• Must and should take the bare verb; have and need take 'to': must go, have to go.
• Use have to for outside rules and must for strong or written rules.
• Should means advice; nobody is forced.
• Mustn't = forbidden. Don't have to = not necessary, your choice.
• For past obligation say had to; for future say will have to.
• Feel the pressure scale: must / have to > need to > should.