~13 min · 12 exercises

Part of the unit “Science: hypotheses, evidence and the limits of certainty”

Modalnost - trebati, valjati, morati i potencijal

modality

Overview

Морати is already known as a fully personal necessity verb (basic-conjunctions-a1, complex-sentences-b1's да-clause machinery built partly on it). This lesson adds требати and ваљати - both impersonal, both near-synonyms of necessity or propriety, but pulling apart sharply in strength and register - and shows how conditionals-b1's би softens all three into the hedged, polite register a C1 speaker needs day to day.

da-constructions-vs-infinitive-c1 already mapped the да-versus-infinitive mechanism that требати's own verbal complement runs on; this lesson assumes that and focuses on what's new here: the three verbs' necessity semantics against each other, морати's second, epistemic life, and the би-softened potential forms built on all three.

Требати - the everyday, neutral necessity verb

In its да-clause/infinitive pattern for necessity to act, требати never conjugates for person - it stays a frozen, impersonal 3rd-singular form no matter who needs to act, exactly like нема (genitive-of-negation-b1). Its verbal complement follows da-constructions-vs-infinitive-c1's subject-based split: a bare infinitive for a fully generic statement (Треба много вежбати. - "One must practice a lot"), да + present agreeing with a specific subject the instant one is named (Треба да вежбаш више. - "You need to practice more").

Требати also has a second, unrelated pattern, for needing a thing rather than needing to do something: a dative experiencer plus a nominative theme, Треба ми одмор. (I need rest). This looks superficially like genitive-of-negation-b1's нема/немати family, but it isn't one - negated, требати still keeps the theme in its base form: Не треба ми твоја помоћ овог пута. (I don't need your help this time), never a genitive *помоћи. genitive-of-negation-b1 already limited its own genitive-of-negation rule to (не)мати/(не)бити specifically; требати simply falls outside that rule, despite the surface resemblance to нема. Unlike the да-clause pattern, this one is not frozen: the nominative theme is требати's real grammatical subject here, so the verb agrees with it in person and number, exactly like any ordinary verb - singular Треба ми одмор. against plural Требају ми паре. (I need money), and just as readily 2nd person when the thing needed is a person rather than an object: Требаш ми. (I need you - literally "you are needed to me"). The увек безлично ("always impersonal") description above is specifically about the да-clause/infinitive pattern; this needing-a-thing pattern conjugates like any ordinary verb, agreeing with its nominative theme in person and number rather than staying frozen.

Ваљати - the literary, proverbial register

The impersonal ваља (from ваљати, to be worth/be fitting) shares требати's basic shape - frozen, impersonal, generally a bare infinitive complement - but sits in a noticeably more literary, formal or proverbial register: Ваља поштовати старије. ("One ought to respect one's elders") reads like a maxim, where Треба поштовати старије. reads like ordinary advice. Ваља combines with a specific да-clause far less naturally than требати does; in practice it stays almost entirely in the generic, infinitive-complement lane.

Negated, не ваља idiomatically broadens well past strict necessity into a general negative evaluation - it's not good, that's improper - not just one shouldn't: Не ваља тако одговарати старијима. can mean either you shouldn't answer your elders like that or, just as often, simply that's not a proper way to answer your elders, with no distinct necessity reading intended at all.

Морати - the strongest necessity, and its epistemic second life

Of the three, морати is the strongest and the only fully personal one - морам, мораш, мора, and so on through every person, the verb basic-conjunctions-a1 already glossed as must - closest to English must/have to. It also does something требати and ваљати essentially don't: read epistemically, as an inference from evidence rather than an obligation. Мора да ради цео дан - телефон му је искључен. (He must be working all day - his phone is off.) states a conclusion, not a duty; compare the deontic Мора да заврши извештај до петка. (He has to finish the report by Friday), a genuine obligation. The surface shape is identical - мора да + present - and only context or surrounding evidence tells the two apart.

Требати and ваљати stay almost entirely in the deontic, advisability lane; neither extends naturally to „it must be the case that…“ the way морати does. That epistemic reach is морати's alone among the three.

Softening all three with the potential

conditionals-b1's би + participle softens any of the three into a hedged, polite suggestion rather than a flat directive: требало би да, ваљало би да, морао би да. Because требати and ваљати are impersonal, their participle never agrees with anything - it stays frozen neuter singular, требало/ваљало, regardless of who is being advised: Требало би да позовеш лекара. ("You should call a doctor"), said to a man or a woman alike. Морати, being genuinely personal, keeps conditionals-b1's ordinary participle agreement: Морао би озбиљније да схвати ситуацију. for a man, Морала би озбиљније да схвати ситуацију. for a woman ("he/she should take the situation more seriously").

This divergence is not an extra rule to memorize - it falls straight out of what each verb already is: an impersonal verb has no real subject for a participle to agree with, so its би-form stays invariant; a personal verb's participle agrees exactly as conditionals-b1 already established for every other verb.

Tables

Three necessity verbs compared
Verb	Strength / register	Person agreement	Example
trebati	neutralno, svakodnevno	nema - bezlično „treba“ (glagolska dopuna); slaže se sa subjektom u obrascu „trebati mi“	Treba da se odmoriš.
valjati	knjiško, izreka, propis	nema - uvek bezlično „valja“	Valja poštovati starije.
morati	jako, obavezujuće	potpuno lično: moram, moraš…	Mora da završi izveštaj.

Examples

Treba da se odmoriš pre puta.

You should rest before the trip.

Specific ти named - да + present, da-constructions-vs-infinitive-c1's own split.

Treba mnogo spavati da bi čovek bio produktivan.

One should sleep a lot to be productive.

Fully generic - the bare infinitive спавати applies.

Treba mi odmor posle ovog projekta.

I need rest after this project.

Требати's other pattern: dative experiencer ми + nominative theme одмор - needing a thing, not doing an action.

Ne treba mi tvoja pomoć ovog puta.

I don't need your help this time.

Negated, the theme помоћ stays in its base form - no genitive shift, unlike нема/немати (genitive-of-negation-b1).

Trebaju mi pare za kiriju do petka.

I need money for rent by Friday.

Требати's needing-a-thing pattern agrees in number with its nominative theme паре (plural) - требају, not the frozen singular треба the да-clause pattern always uses.

Valja poštovati starije, bez obzira na okolnosti.

One ought to respect one's elders, regardless of circumstances.

Ваља + generic infinitive - a more literary, maxim-like register than треба.

Ne valja tako odgovarati starijima.

It's not proper to answer your elders like that.

Не ваља reads as a broad negative evaluation, not narrowly as you shouldn't.

Mora da završi izveštaj do petka, inače će zakasniti na sastanak.

He has to finish the report by Friday, or he'll be late to the meeting.

Deontic мора да - a genuine obligation, with a stated consequence.

Mora da radi ceo dan - telefon mu je isključen.

He must be working all day - his phone is off.

Epistemic мора да - an inference from evidence, not an obligation; the identical surface shape as the deontic reading above.

Trebalo bi da pozoveš lekara ako te i dalje boli grlo.

You should call a doctor if your throat is still hurting.

Softened, invariant требало би - the impersonal participle never agrees with anyone.

Compared with…

English must is one of the closer transfers in the whole course specifically for морати: it carries both the deontic (You must finish this) and the epistemic (He must be home) reading, disambiguated by context exactly the same way as мора да. Should maps loosely onto требало би, though English collapses the impersonal/personal distinction this lesson draws between требало би and морао би into one invariant word. Требати and ваљати, with no strong epistemic use of their own, don't extend as neatly - English should covers ground that Serbian splits across two different, register-marked verbs.

Serbian

Mora da je kod kuće.

English must and Serbian мора да share both readings, deontic and epistemic - a rare near-exact match.

English

He must be home.

Common mistakes

Wrong

Ja trebam da idem ranije danas.

Right

Treba da idem ranije danas.

Требати never takes personal agreement in the standard taught here - it stays frozen треба regardless of person; требам is normal in Croatian but non-standard in Serbian.

Wrong

Ne trebam pomoći danas.

Right

Ne treba mi pomoć danas.

Требати negates simply with не and takes a dative experiencer plus a nominative theme, not a genitive object the way немати does - genitive-of-negation-b1's rule doesn't extend to требати.

Wrong

Treba da vežbati svaki dan.

Right

Treba vežbati svaki dan. / Treba da vežbaš svaki dan.

Да and a bare infinitive never combine in the same clause (da-constructions-vs-infinitive-c1) - pick the generic infinitive or a да-clause naming a subject, never both.

Wrong

Ona morao bi ozbiljnije da shvati situaciju.

Right

Ona bi morala ozbiljnije da shvati situaciju.

Морати is genuinely personal, so its би-participle must agree in gender with the subject - feminine морала for она, not the invariant masculine морао.

Tips & exceptions

Tips

Требати never takes personal agreement in standard Serbian - always frozen треба, with a dative experiencer for needing a thing; resist the Croatian-influenced требам pattern.
Требати is frozen impersonal only in its да-clause/infinitive pattern (Треба да…/Треба + infinitive). In its other pattern, needing a thing, the nominative theme is the real subject and требати agrees with it in person and number: „Треба ми одмор.“ but „Требају ми паре.“ - don't over-apply the always impersonal rule to this second pattern.
To soften a directive or make it more polite, drop a би onto the modal (требало би, морао би) - the exact same потенцијал mechanism conditionals-b1 taught for би + participle, just applied to a modal verb.
Морати is the only one of the three with a natural epistemic „must be…“ reading; требати/ваљати stay in the advisability/propriety lane.

Exceptions

The personal требам agreement pattern is normal in Croatian standard usage but regionally marked or non-standard in Serbian - recognize it, but don't produce it in the standard this course teaches.
Не ваља idiomatically broadens to mean that's not good/that's improper well beyond strict necessity - worth learning as its own fixed expression rather than composing it fresh each time.
Мора да's epistemic reading needs context to disambiguate from the deontic one - a genuinely ambiguous sentence out of context („Мора да ради.“) could mean either he has to work or he must be working; intonation resolves it in speech, surrounding sentences in writing.

Summary

Требати is the everyday, neutral necessity verb - always impersonal, splitting between a bare infinitive (generic) or да + present (specific subject, da-constructions-vs-infinitive-c1) for actions, and a separate dative-experiencer pattern, требати ми, for needing a thing, unaffected by genitive-of-negation-b1's rule. Ваљати shares the impersonal shape but sits in a more literary, proverbial register, and не ваља broadens idiomatically into general disapproval. Морати is the strongest, fully personal necessity verb, and the only one with a natural epistemic must be reading alongside its deontic must do one. conditionals-b1's би softens all three; требало би/ваљало би stay invariant (impersonal), while морао би agrees with its subject like any other participle.