моћи / морати / требати + да + Present-Tense Verb

In contemporary Serbian, modal constructions express degrees of ability, necessity, and advice through finite complement clauses introduced by the conjunction да. With моћи and морати, an infinitive complement is also fully standard, whereas with требати, special restrictions apply.

1. Subject Agreement and Present Tense Rule:
• With моћи and морати, both the modal verb and the main verb carry personal inflection agreeing with the grammatical subject:

• Ми можемо да дођемо. (We can come.)
• Они морају да уче. (They must study.)

• Crucial rule: The verb inside the да-clause is strictly in the present tense (indicative), even when the modal verb is in the past, future, or conditional:

• Могао сам да дођем. (I was able to come — NOT *могао сам да сам дошао).
• Мораћемо да чекамо. (We will have to wait.)
• Могао бих да питам. (I could ask.)

• With требати governing a verbal complement, standard Serbian uses an impersonal construction. The modal verb remains fixed in the 3rd person singular across all tenses and moods (present треба, neuter singular in compound forms like требало је or требало би), while the verb following да inflects for person in the present tense:

• Ја треба да идем. (I should go — NOT *ја требам да идем).
• Ви треба да пазите. (You should watch out.)
• Требало је да дођу. (They should have come.)

2. Infinitive vs. Да + Present:
While да + present is the most frequent and natural pattern across all modals in everyday Serbian, the infinitive complement functions differently depending on the verb:

• With моћи and морати, the infinitive is fully standard and interchangeable with да + present (e.g., можемо доћи, морају учити). The infinitive is especially favored in formal registers and serves to prevent awkward repetitions of да (a stylistic flaw known as дакање): мора почети да ради is preferred over мора да почне да ради.
• With требати, an infinitive complement cannot be combined with a personal subject (do not say *ја требам ићи or *ја треба ићи). Instead, требати + infinitive is strictly generic and impersonal without a subject (e.g., Треба радити поштено = One should work honestly). In a sentence like Треба му помоћи (One should help him), му is not a subject or experiencer of треба, but simply the indirect dative object governed by the verb помоћи (remember: помоћи некоме; constructions like *треба му радити are ungrammatical). Personal dative experiencers with требати occur only when followed by noun complements (e.g., Треба му новац = He needs money).

3. Negation and Nuances:
• Negating морати (не морам да...) cancels obligation, translating as 'I don't have to' or 'I need not'.
• Negating требати (не треба да...) indicates that an action is unnecessary or unadvisable, translating as 'should not' or 'do not need to' (e.g., Не треба да журиш = You don't need to rush / You shouldn't rush).
• To express strict prohibition ('must not' / 'not allowed to'), Serbian does not negate морати; instead, it uses не смети + да (e.g., Не смеш да пушиш овде = You must not smoke here).

4. Clitic Word Order:
• In dependent clauses introduced by да, second-position pronominal and reflexive clitics (се, ми, ти, га, је) attach immediately to да:

• Мораш да се наспаваш. (You must get some sleep.)
• Могу ли да ти помогнем? (Can I help you?)

• When an infinitive is used instead of a да-clause (such as with моћи or морати), clitics place immediately after the finite modal verb rather than after the infinitive: Мораш се наспавати; Могу ти помоћи.