Usage of "is when"

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.

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):

WHEN AND WHERE

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.

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

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.

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.

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.

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."

The Sportsman's Dictionary: Or The Country Gentleman's Companion, In All Rural Recreations [unpaginated]

https://books.google.com/books?id=jy4PAAAAYAAJ

1735

"HIP-SHOT is when [emphasis supplied] the hip-bone of an [sic] horse is removed out of it's [sic] place"

Apparently, is when has been in use for centuries, and not only by children.

Richardson (above) is quite correct: copula verbs join two nouns as 'A debate is a formal discussion', or modify the subject noun as 'The debate was lively.' 'Discussion' answers the question 'What is it?' 'Lively' answers the question 'What was it like?' Choster (above) is quite mistaken about adverbials: they are not nouns. 'Weak' is an adjective modifying 'coffee'. 'Home', in 'He went home', is a common noun, the object of the transitive verb 'to go'. 'He napped this afternoon.' is actually an improper construction, turning the intransitive verb 'to nap' into a transitive one with 'afternoon' as its object. In fact, what's been done is the elision of the preposition that introduces the adverbial phrase which modifies 'nap' in the past tense and answers the question: 'When did he nap?' The answer is quite clearly something along the line of 'He napped during this afternoon.' The adverbial is now quite clear, and not a noun.

The subordinate conjunctions 'when' and 'where' that began this discussion (debate?) as 'A debate is when ...' introduce simple adverbial clauses. However, there is nothing for them to modify since copula verbs are non-modifiable. Hence, we create a nonsense sentence. If a debate is to be defined, select a close cousin (as e.g. 'discussion'), then modify the cousin appropriately: 'A debate is a regulated discussion of a proposition between two matched sides.' (Merriam-Webster's Collegiate Dictionary, 11th edition.)

If a debate is to be described, switch to a describing verb as in: 'A debate occurs when two matched sides discuss a proposition, the discussion being regulated by a moderator.' Then the adverbial clause can properly modify 'occurs'. Schleicher's solution above is a correct one, but for the wrong reason. Yes, one way out of the faulty construction is to insert the additional words, 'taking place'; but these cannot be construed as having been left out. Doing so automatically presumes the original author meant to describe a debate, not define the word itself, which the naked use of the copulative otherwise shows such intent, albeit malformed. Were Schleicher the original author, he's welcome to do as he did; but it's not the only way of describing a debate, as I just showed. However, if he meant to define 'debate', he will need to stick with the copulative, give it a noun to join to the sentence's subject and supply whatever modifications are required for greater specificity.

A simple way to understand the verb 'to be', and to use it correctly, is to think of arithmetic class: 'five and three are eight' ('five and three equals eight') (Why use the plural form of the verb 'is'? Compound subject. But if we meant to say is that 'The sum of five and three is eight.' the singular form is correct.) But 'Five nines are forty-five.', to wit 'Five nines equals forty-five.' with truly a compound subject.

The use of 'to be' in a definition is now clear: it is connecting one and the same thing, albeit, each expressed differently. And 'thing' is clearly what we must have in mind (eight and forty-five being nouns). More abstractly, copulatives connect objects, in the sense of object-oriented programming. Then, for example, we can legitimately say 'Debating with Biden is Trump at his worst.'