This is the first part of the lesson. The rest, the exercises and the reviews are in the app.
The instrumental of the predicate and of time: bio je student, subotom radim
Besides the tool and the companion, the instrumental has two more jobs, and both hold small surprises. The first is the predicate noun and the role: “he became a doctor”, “I consider him a friend”. A predicate noun is the noun that says what the subject is or becomes: in “he is a student”, student is the predicate noun, and “is” is the linking verb (grammarians call it the copula). The second job is time: “on Saturdays”, “by day”, “at night”. We start with the most important point, where a learner fresh from the instrumental is most likely to overdo it. After biti “to be”, the predicate noun stays in the nominative, the plain dictionary form: bio je student (he was a student), not “bio je studentom”. Several other Slavic languages put the instrumental here; Serbian does not, and we will deal with this first.
You have seen the start of this. In “Subject & predicate nominative” the noun after biti stayed in the nominative in every tense (On je bio lekar.), and usually after postati “to become” too (Postao je lekar.). This lesson shows where the instrumental predicate does appear: it is possible with postati and usual with smatrati “to consider”. And in “Instrumental forms” you met the bare instrumental of regular time (jutrom, subotom); here we set it beside u subotu and the other ways of saying “when”.
The predicate noun: why bio je student, not “studentom”
The main rule in brief: after the linking verb biti, the predicate noun is in the nominative. In the present, in the past and in the future alike. However much of the instrumental you have learned, it does not belong here.
In every line the Serbian predicate stays in the nominative: student, lekar, muškarac, just as they appear in the dictionary. Note, too, that there is no word for English “a”: Serbian has no articles, so On je student is the whole sentence. The rule holds for the linking verb in any of its forms: je, bio je, biće, da budem, budi; biti sets two things equal, and both sides stand in the nominative. An instrumental after biti (bio je učiteljem) sounds old-fashioned today; you may meet it in old books. The question word takes the nominative too, just as English asks “what”: Šta želiš da budeš? “What do you want to be?”
Cheat sheet
The cheat sheet — everything essential from the lesson on one page. In the app it opens at any time.
The instrumental of the predicate and of time: bio je student, subotom radim
Two jobs of the instrumental. A predicate noun after biti stays in the nominative, not the instrumental; the instrumental predicate appears only with a few verbs. The second job is time: the bare instrumental means “regularly, every…”.