Intuition: the question remains even though the clause is not pronounced
A specific information gap often arises after a statement. In Someone called, but I do not know who, the final who does not name a person; it asks a dependent question about that person. The full paraphrase who called helps show the meaning, but the form actually used ends with the wh-remnant.
The same mechanism works as a separate utterance: Someone called. — Who? Here, intonation and context give a single remnant the force of a direct question. After I do not know who, the whole sentence is a statement about not knowing, containing a short question within it.
The term sluicing belongs to grammatical analysis. For learning purposes, it is enough to check whether there is a wh-question, whether the context allows its subject matter to be recovered, and whether the unpronounced part is unnecessary for distinguishing several possible meanings.
context with an open detail + (a know / say / explain frame, etc.) + wh-phrase [the interrogative clause is not pronounced]
- clause —
- A construction with a verb, often with its own subject: when I arrived. A non-finite clause may leave the subject unspoken: to get home.
Someone has taken the spare key, but nobody knows who.
Someone took the spare key, but nobody knows who.
Who is the remnant of a question about the agent; the indefinite someone provides an explicit correlate.
The server stopped responding, but we cannot explain why.
The server stopped responding, but we cannot explain why.
The reason is pragmatically recovered from the event, although there is no separate something.
The remnant is a wh-phrase, not necessarily a single word
The remnants can be who, what, when, where, why, and how, but also which report, which of the three, whose proposal, how many applications, how much longer, with whom, and more complex wh-phrases. It is not everything after the first wh-word that is omitted: words that refine the question itself belong to the remnant.
A noun or determiner is often needed for clarity. After A researcher from one university published in one journal, the phrase which one could refer either to the university or to the journal. Which journal or in which journal removes the competition between correlates.
Semantic labels overlap. Which of the three expresses both a choice and a quantitatively restricted set; whose report asks about possession while identifying an item. A teaching category names the focus of a particular exercise; it does not deny other properties.
Several recommendations were adopted, but the minutes do not say which ones.
Several recommendations were adopted, but the minutes do not say which ones.
Which ones preserves the plural number of recommendations.
The review found a fault, but it is unclear how serious.
The review found a fault, but it is unclear how serious.
How serious is the entire gradable wh-phrase; the full paraphrase ends with the fault is.
Recoverability: the context must provide enough information, but not the answer itself
The listener needs to understand which event or relation is being questioned. An explicit indefinite correlate—someone, something, somewhere, one of the reports—makes the connection transparent, but it is not an absolute requirement.
The event itself can open up a natural parameter: The lights went out, but nobody knows why. The reason is not named or presented as something, yet the question “why the lights went out” is easy to recover. In the specialist literature, this related subtype is often described as sprouting; it remains a useful example of a general wh-remnant.
Recoverability does not automatically select a wh-word. The plan changed allows relevant questions such as why, when, how, or in what way, depending on the purpose. A closed exercise can have one answer key only when the visible context specifies the required parameter.
The courier stopped somewhere nearby, but the app does not show where.
The courier stopped somewhere nearby, but the app does not show where.
Somewhere explicitly directs where toward the place of stopping.
The final briefing ended abruptly, but nobody could explain why.
The final briefing ended abruptly, but nobody could explain why.
The question about the reason is recovered from the event without an explicit indefinite noun.
Root and embedded questions use different frames
A separate full question usually has question word order: Where did they put the key? Its shortened dialogue counterpart is Where? The auxiliary did does not remain stranded after the wh-phrase.
After frames such as know, wonder, remember, say, explain, and be clear, a full embedded question has subject → verb order: I wonder where they put the key; nobody explained why the system failed. The forms where did they put and why did the system fail are not appropriate inside such frames.
In the sluice itself, the difference is not always visible because only the wh-phrase is pronounced. That is why expanding it is a useful check: for the standalone Who?, the natural full paraphrase is Who called?, while for I do not know who, it is I do not know who called or who they called, depending on the required role.
A: They stored the archive somewhere. B: Where?
— They stored the archive somewhere. — Where?
A root sluice is a separate direct question.
They stored the archive somewhere, but I do not remember where.
They stored the archive somewhere, but I do not remember where.
Where is part of an embedded question after remember; the whole sentence does not require a question mark.
Grammatical connections and register do not disappear
The form of the wh-phrase depends on the role it would have in the full question. After of, the pronoun must have the object form them: which of them, not which of they. Countable plural nouns combine with how many; uncountable nouns combine with how much.
In English, a preposition can remain together with the wh-phrase or be unexpressed in a short version when that full question is natural. Peter spoke with someone, but I do not know with whom is more formal; ... but I do not know who is common in many conversational contexts. With who also occurs in informal usage. A difference in register should not automatically be called a grammatical error.
Whom marks the object role in more formal style; who is also widely used in the object role. Before a preposition, standard formal to whom / with whom is especially noticeable. When an exercise requires a particular register, it should state this explicitly.
The analyst consulted with someone, but the report does not state with whom.
The analyst consulted with someone, but the report does not state with whom.
With whom explicitly preserves the preposition in a formal register.
Mina spoke to someone after the talk. — Who?
Mina spoke to someone after the talk. — Who?
Informal who is natural as a short question about the object; whom is possible in more formal usage.
Economy is optional, and pragmatic conclusions are limited
The full I do not know why she left and the shortened I do not know why are both grammatical. Sluicing often avoids repetition and makes the wording lighter, but the full form may be appropriate for clarity, contrast, or rhythm. Therefore, “not shortened” is a failure to meet the goal of a particular exercise, not automatically a language error.
I cannot remember when, the report does not say which, and the editor would not disclose who distinguish the holder of knowledge from the availability of information. The report does not say does not imply that the authors do not know; would not disclose does not imply that the information does not exist.
A wh-remnant does not establish a specific answer, time, reason, or agent. It preserves the question. Likewise, may, might, negation, and temporal relations in the context must not be unjustifiably strengthened during shortening.
She resigned, but the announcement does not say why.
She resigned, but the announcement does not say why.
The gap concerns the wording of the announcement, not necessarily the knowledge of all the participants.
She resigned, but the announcement does not say why she resigned.
She resigned, but the announcement does not say why she resigned.
The full form is grammatical and conveys the same basic question; it simply does not use sluicing.
Theory and usage: a useful paraphrase without excessive certainty
An influential structural theory analyzes sluicing as a wh-phrase together with an unpronounced clause and explains why the remnant preserves the grammatical connections of the full question. At the same time, both structural and nonstructural approaches exist in the literature; the detailed nature of the unpronounced material is a theoretical question.
For a learner, full expansion is a practical diagnostic tool: it checks the role of the wh-phrase, tense, modality, and word order. But a formula such as who called should be called a full paraphrase, not the only proven representation in the speaker’s mind.
Several adjacent wh-remnants have variable and often marginal acceptability in English. If Who what? or who whom sounds unnatural, a clear full clause or coordination of two questions is better than maximum shortening. At C2 level, control here means knowing not only how to elide, but also when to avoid over-reduction.
Everyone submitted something, but I cannot reliably tell you who submitted what.
Everyone submitted something, but I cannot reliably tell you who submitted what.
The full form avoids the potentially marginal double remnant who what.
They moved the meeting, but I do not know when or why.
They moved the meeting, but I do not know when or why.
Two coordinated questions are clearer than an artificial sequence of several wh-words.
Common mistakes
Incorrect:The panel rejected the proposal, but nobody explained why did the panel reject it.
Correct:The panel rejected the proposal, but nobody explained why the panel rejected it.
After explained, this is an embedded question, so the subject comes before the verb; inversion with did is ungrammatical here.
Incorrect:Three proposals remain, but the board has not decided which of they.
Correct:Three proposals remain, but the board has not decided which of them.
After the preposition of, the pronoun must have the object form them, even when the rest of the question is not pronounced.
Incorrect:We need more equipment, but nobody can say how many.
Correct:We need more equipment, but nobody can say how much.
Equipment is uncountable in this meaning, so the quantity remnant agrees as how much.
Incorrect:The guard could not remember when had the visitors arrived.
Correct:The guard could not remember when the visitors had arrived.
A full embedded question after remember does not have inversion; the Past Perfect is preserved nonetheless.