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In a grammar class I'm enrolled in, one of the diagnostic tests contained the following question:

"Blocking" is when a television network schedules a less popular program between two popular ones

According to the test, a good revision would be:

"'Blocking' is a television network's practice of scheduling a less popular program between two popular ones."

Why is it grammatically incorrect to connect two subjects with "is when" or "is how", as in the above example? My teacher called it an 'illogical expression.'

Are there any other "is __" phrases that I should avoid?

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up vote 2 down vote accepted

The verb to be is a copular verb which is typically followed by an adjective or noun phrase as its complement:

Blocking is common in modern television.

Blocking is a common television practice.

In your sentence the complement of the verb is a temporal clause starting with when. This is 'illogical' in the same way the following sentences are illogical:

The test is next Wednesday.

The test is when I decide you are ready to do it.

In fact, the test takes place or will be scheduled for next Wednesday or when I decide you are ready.

Similar constructions are:

Cancer is why you shouldn't smoke.

Practice is how you improve.

Careful writers may wish to avoid such constructions, but they are common. An example is the cartoon series "Love is .. ":

Love is when he brings you breakfast in bed.

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I can't accept 'The verb to be is a copular verb which is typically followed by an adjective or noun phrase as its complement' here. A quick look at the AHD or Collins entries shows many other typical usages of be. I am here where be is followed by a locative is about as typical as it gets. (I'd say this is still a link-verb usage; the dictionaries disagree on this point.) Prepositional phrases may be used as complements of link verbs (She stayed in the garden), but I agree that the 'is when' construction sounds clumsy here - The test is when I decide if you are ready is better. – Edwin Ashworth Jun 12 '13 at 7:59
@Edwin. I should have made it clearer that when the verb to be has a subject complement then this is typically a noun or adjective phrase. The perceived problem with the OP's sentence is that it has a temporal clause as its subject complement. But you are right that not all complements are subject complements. – Shoe Jun 12 '13 at 8:29

It is grammatically/ semantically incorrect (esp. in writing) because an important expression is omitted. There's ambiguity.

'Blocking' is (what happens/ that which takes place) when a television network schedules …

It does not mean the same thing as:

'Blocking' is a television network's practice of …

The first case refers to a result of an action, while the second correctly defines the action.

Omitting the expression as in your example can also cause the sentence to be interpreted as

'Blocking' is (what you do) when a television network schedules …

An instance of an action that is performed when a condition occurs.

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Nice analysis - I'd just suggest names rather than defines. – Edwin Ashworth Jun 13 '13 at 9:59
@EdwinAshworth Now you put me in a bind! :) Is it names? Is it defines? The verbs run in opposite directions. 'Blocking' names, the clause following it 'defines', right? The purpose of the sentence could be to either define the term or name the action. The sentence can be used either way. – Kris Jun 13 '13 at 11:46

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