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In grade school, when writing stories for English classes I recall being gently corrected whenever I handed pieces in that contained sentences with a structure similar to this:

“A debate is when a group of people get together and argue for fun,”

or more generally "noun is when description" or "metaphor is when philosophical prose."

I don't recall the justification for not employing is when in sentences, but even now, I find myself avoiding its usage simply because it "feels" clumsy.

Is there any legitimate justification for avoiding this grammar?

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The usage of is when looks fine to me. Sentences using is when are definitions of events. – Dan D. May 2 '12 at 3:01
Even if one could find a universally agreed proof of something being perfectly grammatical, there's no need to use it if it "feels clumsy" to you. That would take someone to show it's grammatical and eloquent compared to alternatives. Being correct is only part of being good. – Jon Hanna Feb 15 '13 at 15:15
What else would you use? I'm having trouble thinking of anything... – Ron Kyle Apr 7 '14 at 10:24

I, too, was taught not to use "is when" and "is where" to define words. I like this entry from "The Careful Writer," by Theodore M. Bernstein (an old book, but worth owning):


One school kid will say, "Addition is when you add two and two." Another will say, "Addition is where you add two and two." Both are using a juvenile construction. Most authorities agree that the construction is undesirable, but they do not agree on why this is so. One advances the theory that when (and presumably where also) cannot be used to join a clause to a noun—there must be two full clauses. It is perfectly proper, however, to say, "Noon is when the sun is directly overhead," and "Home is where Affection calls." Perrin* says rightly that the objection to the when and where clauses as used by juveniles is stylistic rather than grammatical and comes from their overuse in amateurish definitions. He might have gone a step further and said that the stylistic objection arises from the use of when and where in situations where their meanings do not apply. When, for instance has a temporal meaning, and it does not apply in a sentence like this: "The 'hard-ticket policy' is when they up the price of admission to a $3.50 top and sell reserved seats to performances that are given only twice a day." That kind of sentence is not suitable in mature writing.

(*Perrin is Porter G. Perrin, author of "Writer's Guide and Index to English" and a contemporary of Bernstein's. Bold emphasis is mine.)

I have no idea what Mr. Bernstein's last example sentence means (and it is quaint to think of admission to anything being $3.50), but I agree that the objection to this kind of construction is largely stylistic. I suppose your teacher could have been trying to help you develop a more mature writing style.

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There are two questions you could ask: 1. is this acceptable?, and 2. if not, why not?

  1. It is fine in casual writing, but it may not be appropriate in formal writing; it sounds a bit too informal for a formal definition.

  2. I can only speculate, but I suspect that there is a certain discrepancy between the word (debate) and the definition (when...). A debate is an (inter)action, while a clause starting with when describes a point or period of time. It is like saying a debate is a time when a group of people argue: a debate is not really a time. It would be better to use a noun that can be equated to debate, like this:

A debate is a structured exchange of arguments.

A quick way to circumvent this (minor) issue is by loosening up the equation:

When a group of people get together and argue for fun, this is commonly called a debate.

The intervention of this breaks the direct equation. There is no real discrepancy here, because this does not refer to the exact content of the when clause, but to a somewhat vaguer, implicit antecedent. The vagueness still makes it less suitable for a formal definition.

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Both of these answers are excellent and well thought out - credit to both of you for the thoughtfullness of your posts. – Ray May 2 '12 at 16:21
@Ray: Thanks! So are you the same person as the asker of this question? – Cerberus May 2 '12 at 17:59
Yes. I had forgotten to log in. – Ray May 2 '12 at 18:11

In Max Morenberg's Doing Grammar (1991), a widely used synthesis of traditional and modern grammars (widely used in college), he explains that "linking verbs must be followed by a noun or an adjective" (p. 5). "When" and "where" clauses are abverbial. Readers who object to "Debate is when people argue" on stylistic grounds have likely forgotten the rule, but their ears are still attuned to the sound of the mistake. This happens to me all of the time. I teach writing but am getting older; I find that I sometimes forget specific rules but know that a mistake "sounds wrong."

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"Is when" refers to time. "Is where" refers to place.

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I think it is as simple as this: a 'thing' (noun, pronoun, noun phrase or noun clause) cannot logically 'be' an adverb, adverbial phrase or adverbial clause.

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Adverbials are nouns which indicate time/duration, location/direction, value/quantity, or manner, and quite common: The coffee was a bit weak, I napped this afternoon, I went home and napped some more, and so on. – choster Apr 6 '14 at 14:57

The solution to this might be even more simple. When using a construction like:

A debate is when many people discuss the pros and cons of X

necessary parts of the first sentence are omitted, namely parts of the predicate and a possible object. The full sentence should be along these lines:

A debate is taking place when...

Therefore, a construction with the words "is when" creates fragments and not full sentences and is thus stylistically problematic.

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Copula verbs ('linker' verbs like "to be", "to feel", "to seem," et cetera) are normally followed by one of two things: an adjective or a noun:

Eg. Grammar is difficult./Complex grammar is hell.

Copula verbs can be followed by neither 'when' nor 'where' because 'when' and 'where' are both adverbial.

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It's an insteresting theory, but isn't how the world works. – Matt E. Эллен Apr 7 '14 at 7:51
What's wrong with: "Spring is when the azaleas bloom". – Peter Shor May 1 '15 at 21:57
"Spring is when the azaleas bloom" is elliptical for "Spring is the season when the azaleas bloom." I agree with the view that an adverbial phrase cannot properly serve as a subject complement, which must be an adjective, noun, or pronoun. – user149937 Dec 2 '15 at 19:13

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