Infinitives and Da Complements

In Serbian grammar, verbal complementation after modal verbs (such as personal моћи and морати, or impersonal требати), future auxiliary clitics, aspectual verbs, or verbs of intent can take either a non-finite infinitive or a finite да + present indicative clause.

1. Coreferential Subjects with Personal Verbs:
When a personal governing verb shares its subject with the dependent action, both constructions are grammatically acceptable:

• Морам учити (Infinitive) — slightly more formal, abstract, or condensed.
• Морам да учим (Да + present) — immediate, concrete, and standard in everyday speech.

Important distinction with требати: Unlike personal modals (морам учити / морам да учим), modal требати governing a verb is strictly impersonal in standard Serbian. Because an infinitive is non-finite, pairing it with impersonal треба creates a generic, impersonal obligation without a specific actor: треба учити means 'one must study' or 'it is necessary to study'. To express a specific person's obligation, standard Serbian requires the finite да-clause with a conjugated present verb: треба да учим ('I should study'), треба да учиш ('you should study'), never *ја требам учити.

2. Future I (Футур I) Alternation:
Learners frequently encounter this alternation in Future I. Serbian permits both the infinitive construction and the analytical да + present construction:

• Писаћу / Доћи ћу / Ја ћу писати — standard future using the infinitive. When no overt subject pronoun precedes, verbs ending in -ти drop the suffix and merge into a single synthetic word (писати → писаћу), whereas verbs ending in -ћи remain written as two separate words (доћи ћу, рећи ћу), with the full infinitive serving as the prosodic host for the clitic.
• Ја ћу да пишем — future using да + present, very common in spoken Serbian. Crucial morphosyntactic constraint: Because the future auxiliaries (ћу, ћеш, ће...) are clitics, they can never stand in clause-initial position. Therefore, unlike the infinitive forms Писаћу or Доћи ћу, the да-construction cannot stand on its own (*Ћу да пишем is ungrammatical) and strictly requires an overt stressed preceding host—such as a subject pronoun (Ја ћу да пишем), a temporal adverb (Сутра ћу да пишем), or the full tonic verb (Хоћу да пишем).

3. Heteroreferential Subjects (Different Subjects):
If the subject of the main clause differs from the subject of the subordinate verb, the infinitive is strictly blocked. English accusative-and-infinitive structures cannot be translated with an infinitive:

• Желим да дођеш (I want you to come) — correct.
• *Желим те доћи — completely ungrammatical.

4. Clitic Placement in Да-Clauses:
When using the да-construction, any pronominal or auxiliary clitics belonging to the subordinate verb must immediately follow the conjunction да as the second element of the subordinate clause:

• Хоћу да му га дам (I want to give it to him) — clitics cluster directly on да.
• Compare with the infinitive: Хоћу дати му га or Хоћу му га дати.

5. Register and Stylistic Dissimilation:
While colloquial speech relies heavily on да, cultivated literary and journalistic Serbian deliberately alternates between both structures to avoid clumsy stacking (дакање). Stacking multiple да-clauses (морао је да покуша да схвати) is grammatically intelligible but stylistically discouraged; careful writers prefer alternating: морао је покушати да схвати.

6. Aspectual Rules:
After phasal verbs like почети, наставити, престати, the complement verb must always be in the imperfective aspect, whether an infinitive or a да-clause is chosen (e.g., почео је писати or почео је да пише, never *почео је да напише).