Have to, must, should, could
English learners often confuse have to, must, should, and could because they can express obligation, necessity, or possibility. However, they are used in different situations and with different strengths.
1. Have to
Meaning
- Expresses necessity or something that is required, often because of rules, laws, or external factors.
Form
Subject + have/has to + base verb
Examples:
- I have to wear a uniform at work.
- She has to finish her homework before going out.
- We don’t have to pay for water here. (no obligation)
Notes
- “Have to” is more objective. It’s not the speaker’s personal opinion, but a rule or fact.
2. Must
Meaning
- Expresses a strong obligation, often coming from the speaker.
- Also used for strong advice or logical conclusions.
Form
Subject + must + base verb
Examples (obligation):
- You must wear a helmet on this site.
- Students must not cheat in exams.
Examples (strong advice):
- You must try this cake — it’s delicious!
Examples (logical conclusion):
- He must be tired after such a long day.
Notes
- “Must” feels stronger and more personal than “have to”.
- Negative: must not (mustn’t) = prohibition (not allowed).
3. Should
Meaning
- Expresses advice, recommendation, or expectation (weaker than must).
Form
Subject + should + base verb
Examples (advice):
- You should eat more vegetables.
- He should visit the doctor.
Examples (expectation):
- The train should arrive at 9.
- She should be home by now.
Notes
- "Should" is softer, more like a suggestion.
- Negative: shouldn't = it's not a good idea.
4. Could
Meaning
- Expresses possibility, ability (past), permission (polite), or suggestion.
Form
Subject + could + base verb
Examples (possibility):
- It could rain tomorrow.
- She could be at home.
Examples (past ability):
- When I was young, I could run very fast.
- He could speak three languages.
Examples (polite permission/request):
- Could I borrow your pen?
- Could you help me with this?
Examples (suggestion):
- You could try calling him.
- We could go to the movies tonight.
Notes
- "Could" is the past form of "can" but also used for present/future.
- More polite than "can" for requests.
- Negative: couldn't = not possible or not able.
5. Common Mistakes
| Wrong | Correct |
|---|---|
| ❌ I must to go now. | ✅ I must go now. |
| ❌ You don’t must smoke here. | ✅ You must not (mustn’t) smoke here. |
| ❌ He should to study. | ✅ He should study. |
6. Comparison
| Modal | Strength | Use | Example |
|---|---|---|---|
| Have to | Strong, external obligation | Rules, laws, necessity | I have to pay taxes. |
| Must | Very strong, often personal | Strong obligation, advice, deduction | You must wear a seatbelt. / He must be tired. |
| Should | Weak, soft | Advice, recommendation, expectation | You should drink more water. |
| Could | Possibility, polite | Possibility, past ability, requests | It could rain. / Could you help me? |
Quick Review
-
Have to = external obligation (rules, laws, necessity).
-
Must = strong obligation, personal necessity, or logical conclusion.
-
Should = advice, recommendation, or expectation (weaker).
-
Could = possibility, past ability, polite requests, or suggestions.
-
Be careful with negatives:
- don't have to = not necessary.
- must not = prohibited.
- shouldn't = not a good idea.
- couldn't = not possible or not able.
Comparative & Superlative
Comparative and superlative adjectives are used to compare people, things, or situations. Comparatives compare two things, while superlatives compare one thing to all others in a group.
The Passive Voice
The passive voice is used when we want to focus on the action or the object that receives the action, rather than on who performs the action. It's formed with "be" + past participle.