In short

To turn any polite sentence into a question, add か (ka) at the end and change nothing else - no inversion, no extra helper verb, no reordering. 学生です。 (Gakusei desu., You are a student.) becomes 学生ですか。 (Gakusei desu ka., Are you a student?). To ask what, who, where and so on, drop the question word into the exact slot where the answer would go, and still finish with か.

The rule

Step 1 - Add か and leave the rest alone

Japanese does not move words around to make a question. English swaps the subject and verb (You are -> Are you) or bolts on do (You eat -> Do you eat). Japanese does neither. The pattern is simply:

Statement + か

• 田中さんです。 (Tanaka-san desu.) It’s Mr Tanaka. -> 田中さんですか。 (Tanaka-san desu ka.) Is it Mr Tanaka?
• 食べます。 (Tabemasu.) I eat. -> 食べますか。 (Tabemasu ka.) Do you eat?

This works on every polite ending: the です, じゃないです, でした you already know, and the verb endings ます, ません, ました, ませんでした that arrive in Verb groups and the polite ます-form. In speech the sentence ends on a rising intonation, but か is doing the grammatical work, not the pitch.

Step 2 - The question mark is optional (か already marks it)

Because か marks the question, standard written Japanese ends a question with the ordinary full stop 。, not ？. So お元気ですか。 (O-genki desu ka.) is correctly punctuated. You will still see ？ constantly in casual writing, messages and manga - especially when か is dropped and only intonation remains (元気？ Genki?). Keep か in polite speech; leaving it off sounds blunt or overly familiar.

Step 3 - Put the question word in the answer’s slot

An interrogative word is not fronted to the start of the sentence the way English fronts what and who. It sits exactly where the answer would sit:

• 駅はどこですか。 (Eki wa doko desu ka.) - Where is the station? Answer: 駅はあそこです。 (Eki wa asoko desu.)
• 何を食べますか。 (Nani o tabemasu ka.) - What will you eat? Answer: すしを食べます。 (Sushi o tabemasu.)

Swap the answer for the question word and you have the question. That one habit removes most beginner word-order errors.

Step 4 - 何 is read なに or なん depending on what follows

The kanji 何 has two readings. It is なん (nan) before です, の, で, と, and before every counter; it is なに (nani) elsewhere, notably before を and が. The mnemonic: なん before t-, d- and n-sounds, and before counters.

One word carries two meanings inside that rule: 何 + で is read なんで (nande) either way. Dictionaries give なんで both why and by what means, so the reading does not separate the two - 何で行きますか。 (Nan de ikimasu ka.) is how will you go? when the topic is travel and why will you go? when it is a reason. Context decides. To force by what means, use どうやって (dō yatte, how, in what way); some speakers also pronounce it なにで (nani de) to keep them apart, but that is a spoken habit, not a dictionary reading.

Step 5 - A question word as subject takes が, never は

This is a hard rule with no exceptions: 誰が, 何が, どれが, どちらが - never 誰は, 何は. は marks a topic, that is, information already shared; a question word is by definition unknown, so it cannot be the topic.

• 誰が来ますか。 (Dare ga kimasu ka.) - Who is coming?

The answer keeps が too: 山田さんが来ます。 (Yamada-san ga kimasu.) A question word can still appear after は elsewhere in the sentence, because there は belongs to a different, known element: あなたは何を飲みますか。 (Anata wa nani o nomimasu ka.) Here あなた is the topic and 何 is still the unknown. See Topic は and subject が.

Step 6 - Answering: はい／いいえ and そうです

For a yes/no question built on a noun + です, answer with はい、そうです。 (Hai, sō desu., Yes, that’s right.) or いいえ、そうじゃないです。 (Iie, sō ja nai desu., No, that’s not right.) The stiffer いいえ、違います。 (Iie, chigaimasu., No, that’s wrong.) also works.

Crucially, そうです answers only です-sentences. A question about an action is answered by repeating the verb: 飲みますか。 -> はい、飲みます。／いいえ、飲みません。

Step 7 - Echo questions and polite deflection

Repeating what you just heard, plus か, asks for confirmation - the equivalent of “Tomorrow, you say?”:

• A: 会議は明日です。 (Kaigi wa ashita desu.) B: 明日ですか。 (Ashita desu ka.)

And a question you would rather not answer with a flat no is deflected by trailing off with ちょっと… (chotto…, literally a little…). 明日はちょっと… (Ashita wa chotto…) is understood as a refusal, and is far more natural than いいえ.

Forms & patterns

Statement to question - the ending never changes shape:

Note that 行きませんか。 (Ikimasen ka.) is normally heard as an invitation (Won’t you come?) rather than a plain negative question - see Expressing desire, invitations and offers. In the same way 学生じゃないですか。 usually lands as a confirmation, He’s a student, isn’t he?, rather than a neutral Is he not a student? - じゃないですか is the everyday tag question.

The interrogative inventory:

何: なに or なん?

Examples

Common mistakes

• 誰は来ますか。 (Dare wa kimasu ka.) -> 誰が来ますか。 (Dare ga kimasu ka.) - a question word as subject always takes が; は marks known information, and an unknown cannot be known.
• なにですか。 (Nani desu ka.) -> なんですか。 (Nan desu ka.) - 何 is read なん before です, の, で, と and all counters.
• 「飲みますか。」「いいえ、そうじゃないです。」 -> 「いいえ、飲みません。」 (Iie, nomimasen.) - そうです／そうじゃないです answer only noun + です questions; a verb question is answered by repeating the verb.
• この傘はいくつですか。 -> この傘はいくらですか。 (Kono kasa wa ikura desu ka.) - いくら asks a price or amount of money; いくつ counts objects or asks someone’s age.
• ですか学生。 -> 学生ですか。 (Gakusei desu ka.) - か is a sentence-final particle and can only stand at the very end; nothing is fronted the way English fronts are or do.

Tips

• There is no “do” and no inversion. Whatever the statement is, the question is that same statement plus か. If you catch yourself reordering words, you are still thinking in English.
• Question word goes where the answer goes. Say the answer to yourself, then swap in 何 / 誰 / どこ - the slot and the particle stay put.
• どうして vs なぜ: both mean why; どうして is the everyday spoken choice, なぜ is formal and written. Casual 何で (nande) also asks why, but sounds blunt - avoid it with strangers.
• どちら is the polite どこ. In service situations you will hear どちらですか rather than どこですか, and どちら様ですか for who are you? - a gentler register of the same question.