може се / мора се

1. Meaning and Function
The constructions може се and мора се are impersonal modal expressions. They remove the human agent from focus and present an action as universally applicable, objectively possible, or societally required. In English, they equate to 'one can / you can / it is possible to' and 'one must / it is required to / you have to'.

2. Syntax and Complements

• Dependent Infinitives vs. да-Clauses: When се directly attaches to the modal verb, може се / мора се strictly governs an infinitive: Мора се радити ('One must work'), Може се рећи ('One can say'). If a clause is structured with да + present, the particle се obligatorily attaches to the subordinate present-tense verb rather than the modal: Мора да се ради, Може да се каже. Combinations such as *мора се да ради or *мора се да се ради are entirely ungrammatical in standard Serbian.
• Clauses Governed by the Infinitive: Impersonal може се / мора се cannot govern a factual content clause directly (*Мора се да је... is ungrammatical). Instead, the modal governs an infinitive of speech, thought, or perception (such as рећи, нагласити, закључити), which then introduces the dependent clause: Мора се нагласити да је време ограничено ('It must be stressed that time is limited').
• Clitic Position and Negation: The reflexive particle се is a pronominal enclitic that follows strict second-position rules (Wackernagel's Law). Although the negative particle не forms a proclitic unit with the verb (не може, не мора), се remains in second position. If a sentence begins with the negated verb, се follows it (Не може се ући), but whenever another constituent opens the clause, се precedes the negated verb: Овде се не може спавати, То се не мора прихватити.

3. Past Tense Formation (Perfekt)
In the past tense (перфекат), impersonal modals take the neuter singular form ending in -ло: могло се, морало се. Importantly, according to the standard rules of се у перфекту, the 3rd person singular auxiliary clitic је is obligatorily omitted before се: Морало се ћутати (never *Морало се је ћутати), О томе се могло разговарати.

4. Impersonal vs. Reflexive Passive Agreement
Pay close attention to whether the dependent verb has a direct object:

• When the dependent verb is intransitive or has an adverbial or prepositional complement, the modal stays strictly in the 3rd person singular neuter/unmarked form: Овде се мора ћутати ('One must remain silent here').
• When followed by a transitive verb with an expressed patient noun, the patient is promoted to the grammatical subject in a се пасив structure. This demands agreement in number: singular Може се видети резултат vs. plural Могу се видети резултати ('Results can be seen').

5. Register
These patterns are ubiquitous across contemporary Serbian. They are used informally for general life rules and instructions (Овде се не може прећи улица), as well as extensively in formal, journalistic, and academic writing to preserve scholarly detachment and objectivity (Као што се може уочити...).