In Spanish we use the impersonal pronoun se with the modal verbs deber and poder to express "you must [do something]" or "you can [do something]" in an impersonal way. In English you could also say "one must" or "one can" but this is more formal.
Se debe + infinitive of main verb
Se puede + infinitive of main verb
The modal verbs will always take the 3rd person singular form of the present.
This type of impersonal sentence where the subject is not specific but rather refers to people in general is used with intransitive verbs, i.e. verbs with no direct objects, like dormir, ir, descansar, pasear...
Have a look at the following examples:
En esta aldea tan tranquila se puede descansar estupendamente.In this quiet village you can have a fabulous rest. (impersonal: one can rest)
Para tener una buena relación con la pareja se debe dialogar mucho.To have a good relationship with your partner you need to talk to each other a lot. (impersonal: one needs to talk)
There are transitive verbs that may be used sometimes with no mention of a direct object. In this case the impersonal form "se debe/se puede" can also be used.
Read and listen to these examples:
En mi colegio se puede leer en silencio en la biblioteca.In my school you can read in silence in the library. (impersonal: one can read)
No se debe fumar para gozar de buena salud.You must not smoke to enjoy good health. (impersonal: One must not smoke)
Remember all modal verbs are followed by an infinitive.
Although this type of impersonal sentence seems very similar to the "passive se" sentences, this type cannot be turned into a "normal" passive sentence with "ser + participle".
No se debe fumar. (You must not smoke)
Fumar no es debido.
See also Forming the Spanish passive se with modal verbs deber/poder (la pasiva refleja)
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