бити

The present forms of the verb бити (to be) perform two critical roles in Serbian: linking a subject to a noun, adjective, or adverbial and prepositional phrases indicating location or state (e.g., Марко је у школи, Они су овде), and functioning as auxiliary verbs in compound tenses.

1. Short (Enclitic) Forms
In standard affirmative statements, the unstressed short forms (кратки/енклитички облици) are most commonly used:

• 1st person singular (I): сам
• 2nd person singular (you): си
• 3rd person singular (he/she/it): је
• 1st person plural (we): смо
• 2nd person plural (you all / polite you): сте
• 3rd person plural (they): су

2. Full (Stressed) and Negative Forms
Serbian also has full, stressed forms (пуни/наглашени облици): јесам, јеси, јесте (or јест), јесмо, јесте, јесу. These are used when emphasis is required, in short affirmative answers (Јеси ли ту? — Јесам.), or when forming questions starting with the verb (Јесте ли спремни?).
Negative present forms fuse with negation into a single word: нисам, ниси, није, нисмо, нисте, нису (as covered in verb negation).

3. Word Order and Sentence-Initial Forms
The short forms (сам, си, је...) are enclitics. In declarative statements, they do not carry sentence stress and cannot appear as the first word; they must lean on a preceding stressed word or constituent (e.g., Ја сам студент or Студент сам, but never *Сам студент).
In yes-no questions with ли that start directly with 'to be', the stressed full forms are generally used (Јеси ли ту?, Јесте ли добро?). However, there is one important standard exception: the 3rd-person singular uses the short form је before ли (Је ли он овде?, Је ли Марко лекар?), while *Јесте ли он овде? is not used.

4. Omission of Pronouns
Serbian is a pro-drop language. As explored in leaving out a clear subject, subject pronouns (ја, ти, ми, ви) are omitted by default when the verb ending makes the subject obvious. For example, Студент сам or Лекар сам are completely natural on their own. In contrast, 3rd-person pronouns (он, она, оно, они...) are typically retained when introducing a referent to avoid person or gender ambiguity, becoming omitted only once the subject has been clearly established.

5. Obligatory Copula in Standard Predicative Sentences
Unlike Russian or Arabic, standard Serbian requires an explicit present-tense copula in ordinary descriptive and predicative statements. To say 'Marko is a doctor', you must include је: Марко је лекар (omitting it is restricted to specific contexts like newspaper headlines, proverbs, or specific grammar rules like reflexive past tenses with се in the perfect).

6. Note: The Perfective Present (будем, будеш...)
Uniquely, the verb бити also has a separate perfective present paradigm (свршени презент: будем, будеш, буде, будемо, будете, буду; see perfective present of be). While сам, си, је... functions as the ordinary copula and general present tense, the будем paradigm cannot stand alone as a simple main-clause copula; instead, it appears in dependent clauses (such as conditionals with ако/када, purpose clauses with да, and in the future-subordinate tense futur II).