Who Ate the Cake? は and が in a Small Office
At a small office, a cake that was in the refrigerator is gone. Three people were in the office: Tanaka, Mori, and Sato. The office manager asks two questions. First: 「だれが ケーキを たべましたか。」 Mori answers: 「たなかさん（ ）たべました。」 Second, the manager turns to Tanaka and asks: 「たなかさんは ケーキを たべましたか。」 Tanaka answers: 「いいえ、わたしは たべませんでした。」 A new employee, Lee, is writing the dialogue down and must fill in the blank. Lee has learned that both は and が can mark a subject, so he is unsure. He also notices that the two questions use different particles and wonders whether the answers should mirror the questions.
1. Fill the blank in Mori's answer and explain why that particle, rather than the other, is correct. Refer to what the manager knows and what he is asking for.
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The blank takes が: 「たなかさんが たべました」.
The manager asks 「だれが」, a question that requests new information: which person did it. The answer must supply that information in the same focus position, and が marks the person as the answer to 'who'. Using は would mark Tanaka as an already-established topic, as if the manager had already been talking about Tanaka. That does not answer the question; it sounds like 'as for Tanaka, he ate it', which presupposes the very thing the manager is trying to find out. With three candidates and no established topic, が is required.
2. Would 「わたしが たべませんでした」 be acceptable as Tanaka's answer? Explain the nuance and say which form fits the manager's yes/no question.
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It is grammatically possible but sounds contrastive: 'I am the one who did not eat it', implying that someone else did. That is a different message from a simple denial.
The manager asked a yes/no question with は: 「たなかさんは ケーキを たべましたか。」 Tanaka is already the topic, so the natural answer keeps は: 「わたしは たべませんでした」. This simply denies the action for the established topic. が would shift the focus onto Tanaka as the answer to an unasked 'who' question. The choice depends on information structure, not on politeness.
3. Mori later says 「ケーキは たなかさんが たべました」. Is this correct, and how does its focus differ from 「たなかさんが ケーキを たべました」? Use this to explain why Lee's 'copy the question's particle' rule fails.
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It is correct. は marks the cake as the topic: 'as for the cake, Tanaka ate it.' が marks Tanaka as the one who did the action. The sentence answers 'what happened to the cake?' rather than 'who ate the cake?'. In 「たなかさんが ケーキを たべました」, the focus is on Tanaka as the doer, with the cake as neutral information.
Lee's rule fails because particle choice is driven by information structure. The manager's first question uses が and the second uses は, and each answer selects its particle to match what is new versus what is already established. Copying the question's particle mechanically would sometimes produce a strange focus.