кад / када + Time Clause

The conjunctions кад and када are interchangeable doublets; the final -а is a mobile vowel (покретни самогласник). Speakers often favor када before consonants like д, т, к, г simply because it flows more smoothly (e.g., када дођеш), but both are fully standard.

1. Past Events:
When narrating past actions, use the ordinary past tense. Use an imperfective verb for an ongoing background action, or a perfective verb for a completed event that sets off the next one:
• Када сам стигао, воз је већ отишао. (When I arrived, the train had already left.)

2. Present and Habitual Events:
To express present circumstances, habits, or recurring events ("whenever" / "every time that"), you can use both aspects in the present tense:
• Imperfective present describes an ongoing background state or activity: Кад пада киша, обично седимо код куће. (When it rains, we usually sit at home.)
• Perfective present (habitual present) is frequently used with кад and кад год to describe recurring complete actions or general truths: Кад год дође, донесе поклоне. (Whenever he comes, he brings gifts.)

3. Future Events (Important Rule):
In temporal adverbial clauses referring to the future, Serbian does not use ordinary future tense (futur I). Instead, verb aspect and verb type determine the form:
• Imperfective verbs: You must use futur II because an imperfective present would signify actual current time: Кад будем имао времена, прочитаћу књигу. (When I have time, I will read the book.)
• Perfective verbs: For a completed future event, you can use either the perfective present (most common in conversation) or Futur II: Јавићу се кад стигнем or Јавићу се кад будем стигао (I will get in touch when I arrive).
• The verb бити: In future temporal clauses, бити typically takes its perfective present form: Кад будем у Београду, посетићу те. (When I am in Belgrade, I will visit you.)

Caution: This restriction against Futur I applies strictly to temporal adverbial clauses. When кад introduces an indirect question (e.g., Не знам кад ће доћи — I don't know when he will come) or a relative clause (e.g., Чекам дан кад ће се вратити — I am waiting for the day when she will return), Futur I is completely regular and required.

4. Contrasts With Other Time Conjunctions:
• Use кад for a general point or moment in time.
• Use док for continuous duration ("while") or док не for boundary points ("until").
• Use чим for immediate succession ("as soon as").