Obligation
Does someone need to do it?
Significado
Obligation shows that an action is required or necessary. Advice is weaker: it says what would be best, rather than what is required. Languages use special verb forms or separate words and phrases, and some have no special obligation form. Permission concerns what someone is allowed to do, not what they need to do.
Como identificar
Look for what makes an action necessary or expected, and check whether the speaker presents it as a firm requirement or weaker advice.
Na tua língua materna
Ter que or ter de plus an infinitive, the verb form used to name an action, expresses a requirement; ter que is especially common in everyday speech. Precisar also expresses a need, with de before a noun and often no de before an infinitive. Dever can express a duty or, depending on context, softer advice.
Tenho que buscar meu filho às cinco.
A gente precisa sair mais cedo amanhã.
Você não precisa trazer toalha.
Bringing a towel is unnecessary, not forbidden.
Na tua língua de aprendizagem
English uses must for a strong requirement, have to and need to for necessity, and should mainly for advice. Must and should are followed by the verb's base form, such as go rather than goes, without to. Don't have to means there is no need, while mustn't means something is not allowed.
You must wear shoes in the gym.
Must presents this as a firm rule.
You don't have to come tomorrow.
Coming is not necessary, but it is not forbidden.
You should get some rest.
Should gives advice rather than a firm order.
Learners whose language uses an infinitive after its word for need may add to after must or should. Learners whose language simply negates that word to remove a requirement may mistakenly read mustn't as no need.