か

か is the primary question-forming particle in Japanese.

Functions

1. Yes/No questions: 行(い)きますか (Are you going?)
2. WH-questions: 何(なん)ですか (What is it?)
3. Alternative questions: Aか B (A or B)
4. Embedded questions: …かどうか (whether…)

Common patterns

• Sentence + か: direct question
• A か B: A or B
• Verb/Adj + か + どうか: whether or not
• Question word + か: embedded question

WITH POLITE VS. PLAIN FORMS: か is standard after です and ます forms, where it is the normal, neutral way to ask a question. In a direct question after plain forms it is much less common in everyday conversation and can sound blunt, literary, or challenging (行(い)くの? is the usual casual question instead). Embedded questions take plain forms with か as a matter of course: 何(なに)を買(か)うか決(き)めた (I decided what to buy).

AかB VS. AかBか: with two nouns, コーヒーか紅茶(こうちゃ) is the ordinary pattern; repeating か after the second option (行(い)くか行(い)かないか) is normal when the alternatives are clauses, and sets up a choice the listener is asked to resolve.

Note: In casual speech, か may be omitted and replaced by rising intonation alone. In very casual male speech, か can sound blunt.