1. Modals of obligation
2. How is must used?
- Must can only be used in present.
Form Examples AFFIRMATIVE Subject + must + infinitive I must study every day for my exam.
You must be polite with the guests.
NEGATIVE Subject + must not / mustn't + infinitive You mustn't smoke here!
You mustn't shout at your brothers.
INTERROGATIVE Must + subject + infinitive ...? Must I do the wahshing-up?
Must we go to school?
- Must is used to express that something is essential or necessary from the speaker's point of view. The obligation is based on the speaker's opinion.We must go home now. It's late.I must give my mom a call.
- Must also expresses a strong obligation that comes from a law or regulation, which if we do not respect, it will lead to a penalty or punishment.You must wear your seat belt.You must turn off all electronic devices.
- The negative form mustn't is used to express prohibition.You mustn't touch the jellyfish.You mustn't tell lies.
3. How is have to used?
- Unlike the modal verb must, which only admits obligation in the present tense, have to admits obligation in all verb tenses.
Structure Examples PRESENT + Subject + have / has to + infinitive You have to do sport every day. - Subject + don't / doesn't + have to + infinitive You don't have to do sport every day. ? Do/Does + subject + have to + infinitive ? Do you have to do sport every day? PAST + Subject + had to + infinitive I had to go to the hospital yesterday. - Subject + didn't + have to + infinitive I didn't have to go to the hospital yesterday. ? Subject + did + subject + have to + infinitive ? Did you have to go to the hospital yesterday? PRESENT PERFECT + Subject + have/has + had to + infinitive I have had to buy a car. - Subject + haven't / hasn't + had to + infinitive I haven't had to buy a car. ? Have / Has + subject + had to + infinitive ? Have you had to buy a car? FUTURE + Subject + will + have to + infinitive You will have to walk the dog tomorrow. - Subject + won't / have to + infinitive You won't have to walk the dog tomorrow. ? Will + subject + have to + infinitive ? Will I have to walk the dog tomorrow?
In negative and interrogative sentences the verb have to always needs the corresponding auxiliary. Don't use only the verb have.You don't have to wash your hair daily.You haven't to wash your hair daily.Did you have to work on Saturday?Had you to work on Saturday?
- Have to is used to express obligation in affirmative sentences. In negative sentences it expresses absence of obligation.I'll have to wear a tie in my new job.You don't have to take the jacket.
4. Differences between have to and must
- In affirmative sentences in the present, both must and have to are interchangeable in many cases. However, the use of have to is preferred when the obligation is external, objective. In contrast, we use must when the obligation comes from the speaker.I have to pick up my brother from school.I must finish the column today.
- In negative sentences, unlike mustn't which denotes prohibition, have to expresses absence of obligation or no need to do something.You didn't have to bring anything.We won't have to clean the car, it'll rain.
In British English, the form have got to is commonly used instead of have to in the present simple tense.
Must and have to indicate obligation, prohibition or necessity.
|MUST||+||Sujeto + must + infinitivo||We must go to the hospital to see Marta's baby.|
|-||Subject + mustn't + infinitive||You mustn't go out when we're doing class.|
|?||Must + subject + infinitive||Must I do everything myself?|
|HAVE TO||+||Subject + have to (correct tense) + infinitive||I've lost my wallet. I have to find it.|
|-||Subject + negative auxiliary (correct tense) + have to + infinitive||You won't have to take anything home.|
|?||Auxiliary (correct tense) + subject + have to + infinitive||Did I have to come earlier?|