Obligation
Does someone need to do it?
Meaning
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.
How to spot it
Look for what makes an action necessary or expected, and check whether the speaker presents it as a firm requirement or weaker advice.
In your native language
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.
In your learning language
Spanish uses tener que and deber with an infinitive, the verb's dictionary form, to express a personal requirement or duty. Hay que with an infinitive presents a general requirement without naming who must carry it out. Debería and deberías often give advice rather than impose a firm obligation.
Tengo que recoger a Lucía a las cinco.
I have to pick up Lucía at five.
Tengo identifies the person who has the obligation.
Hay que validar el billete antes de subir.
You have to validate your ticket before boarding.
Hay que states a general rule, not one particular person's obligation.
Deberías descansar un poco.
You should rest a little.
Deberías makes this advice rather than a firm requirement.
Learners whose language uses one unchanging word for necessity may confuse personal tener que with general hay que. Learners whose language allows a fully formed verb after that word may produce *Hay que estudias instead of Hay que estudiar.