когда

The word когда is the most common way to ask about time in Russian. Note that когда asks about general time (day, month, year, or time of day), while во сколько is used specifically to ask for the exact clock time.

1. Sentence Structure:
In a standard question, когда usually comes first. Because Russian often omits the verb 'to be' in the present tense (see nominal predicate without present copula), you can simply place когда before a noun to ask when something is.
Example: Когда обед? (When is lunch?)

2. The Future Tense:
When когда refers to a future event in a subordinate clause, you must use the future tense. While English says 'When he arrives' (present), Russian strictly requires the future: 'Когда он придёт'. Using the present tense here would change the meaning to a habitual action.

3. Aspect in the Future:
The type of future tense depends on the action. Use the perfective future for one-time results (Когда он придёт?) and the compound imperfective future if the action is an ongoing process (Когда ты будешь работать?).

4. The Past Tense:
Когда is frequently used to anchor past events, often translating to 'when' or 'back when'.
Example: Когда я был маленьким, я жил в Москве. (When I was little, I lived in Moscow.)

5. Habitual Actions:
When an action happens regularly, когда acts like 'whenever' or 'every time'.
Example: Когда идёт дождь, мы сидим дома. (Whenever it rains, we stay home.)

6. Когда vs. Если vs. Пока vs. Пока не:
It is important to distinguish когда from similar words. Use когда for events you are certain will happen. If there is doubt ('if'), use если. To say 'while' (two actions happening at the same time), use пока. Finally, to say 'until', use пока не (literally 'while not').

7. The Correlative Тогда:
Sometimes когда is paired with тогда (then) in the main clause to emphasize the sequence or result.
Example: Когда он придёт, тогда мы поговорим. (When he arrives, then we will talk.)

8. Indirect Questions and Punctuation:
Когда is also used to connect indirect questions (when you report a question inside another sentence). When когда introduces a subordinate clause or an indirect question, that clause must be separated from the main part of the sentence by a comma.
• Я не знаю, когда он придёт. (I don't know when he will arrive. - Comma before the clause)
• Когда он придёт, я уйду. (Comma after the clause)

9. Когда + Infinitive:
To ask 'When should I...' or 'When is it time to...', you can use когда followed by an infinitive, often with a dative pronoun.
Example: Когда мне позвонить? (When should I call?)

10. Related Words:
From когда, Russian forms several important related words:
• никогда (never): Я никогда не сплю днём. (I never sleep during the day.)
• когда-нибудь (sometime, ever): Ты когда-нибудь был в Москве? (Have you ever been to Moscow?)
• когда-то (once, formerly): Он когда-то жил здесь. (He used to live here once.)