quando

In Portuguese, quando is traditionally classified as a temporal relative adverb or a temporal conjunction (conjunção subordinativa temporal). It introduces clauses that provide information about time.

1. The 'Free' or 'Condensed' Relative:
This is a primary standard use of quando. It functions without an explicit noun before it, implying 'the time at which'.
Example: Gosto de músicas de quando era jovem. (I like songs from [the time] when I was young).

2. Explicit Antecedents:
After an explicit temporal noun (antecedent) like dia (day) or ano (year), Portuguese prefers em que or no qual. This is the traditional norm in both Brazil and Portugal, and it is the safe choice in writing.
Preferred: O dia em que nos conhecemos foi inesquecível. (The day when we met was unforgettable.)
Better to avoid in writing: O dia quando nos conhecemos...

English and Spanish both allow their own version here (the day when, el día cuando), so this is an easy habit to carry over. Some Brazilian grammarians do describe quando as a relative adverb that can take an antecedent, and you will hear it in speech, so treat it as informal rather than impossible.

3. Prepositional Usage:
You can combine quando with prepositions to define the timeframe, which you will encounter most frequently in interrogative contexts (note that in these cases, quando functions technically as an interrogative word rather than a relative one):

• Desde quando: Since when
• Até quando: Until when
• De quando: From when
• Para quando: For when

4. Mandatory Mood Selection:
To become proficient in Portuguese, you must choose the correct mood based on the nature of the event:

• Indicative: Used for habits, general truths, or completed past facts. For present habits and general truths, use the Present Indicative. For past habits, use the Pretérito Imperfeito. For completed past facts, use the Pretérito Perfeito.
  Quando eu chego em casa, descanso. (Present habit)
  Quando eu morava no Brasil, ia à praia. (Past habit - Imperfeito)
  Quando ele chegou, a festa acabou. (Past fact - Perfeito)
• Future Subjunctive: If the event is in the future relative to a present or future main clause, use the future subjunctive. (Note: For purely temporal clauses, in standard everyday usage, do not use the Present Subjunctive after quando, a common pitfall for Spanish speakers. Additionally, English speakers must avoid using the Future Indicative here—e.g., never say *Quando eu chegarei). You can also use the compound form (quando + tiver + particípio) for a future action completed before another begins.
  Quando ele chegar, avise-me. (Future event)
  Quando você tiver terminado o trabalho, avise-me. (Completed future event)
• Imperfect Subjunctive: If the main clause is in a past tense (such as the Pretérito Imperfeito) or conditional (Futuro do Pretérito), use the imperfect subjunctive for a future-in-the-past event.
  Ele pediu que eu avisasse quando ele chegasse. (Future relative to the past)
• Indirect Questions (Indicative/Conditional): When quando acts as an interrogative word in an indirect question, it does not trigger the subjunctive for future events. Instead, use the indicative or conditional.
  Não sei quando ele chegará / chega. (I don't know when he will arrive)

5. Reduced Participle Clauses:
In formal or journalistic Portuguese, quando is frequently used directly with a past participle to form a reduced clause.
Quando questionado, ele não respondeu. (When questioned, he didn't answer.)

6. Non-Temporal Uses:
Quando can also be used in conditional ('If/Whenever') and concessive ('Even though/While') contexts.
Conditional: Quando chove, fico em casa. (Whenever it rains, I stay home.)
Concessive: Ele comprou o carro, quando sabia que não tinha dinheiro. (He bought the car, even though he knew he had no money.)

7. Punctuation Rules:
When a quando clause begins a sentence, it is separated from the main clause by a comma. When it comes after the main clause, the comma is generally omitted.
Quando cheguei, ele saiu. (Comma used)
Ele saiu quando cheguei. (No comma)