According to the Bedford Handbook by Diana Hacker, sixth edition, the following constructions must be avoided:
- is when
- is where
- reason ... is because
Here is the exact text:
"In formal English many readers object to is when, is where, and reason ... is because constructions on either grammatical or logical grounds. Grammatically, the verb is (as well as are,was, and were) should be followed by a noun that renames the subject or by an adjective that describes it, not by an adverb clause beginning with when, where, or because. Logically, the words when, where, and becuase suggest relations of time, place and cause -- relations that do not always make sense with is, was, or were." (page 158)

Then she cites Robert Frost on page 195 to show how punctuation is used for emphasis:
The middle of the road is where the white line is -- and that's the worst place to drive.

Look at "is where" in Robert Frost's citation!!! So is this rule about "is when, is where, reason ... is because" something of the past, or something that even good writers don't really heed? And so English teachers should not circle this construction in red ink!?

Thank you.
I find the rule quite peculiar! I have never heard it. This is where it happened is certainly a correct way to be a little emphatic if one isn't contented with It happened here. Maybe you should burn the handbook? Emotion: wink

No, that's the type of use that is frowned on.

If you're going to describe a place as (where it happened, where I grew up, where the Gettysburg Address was delivered), then "this is where..." is just fine.

But if you're using it in place of something like "Dyslexia is a condition in which people..." then it's wrong.
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Gee, it looks like a contradiction, doesn't it! You make a rule and then give an example of breaking it.
Well, not really.
The middle of the road is where the white line is -- and that's the worst place to drive.
"where the white line is" is a clause, "where" is a pronoun (the place in which), and the clause modifies "middle". So I would call it an adjective clause, not an adverb clause. So OK, it does not violate the rule.
Example of adverb clause:
I like to go where my friends hang out.
CoolBreeze, the book is definitelyh one of the best. It's used in many writing centers at US. Universities.

Thanks AlpheccaStars for trying to clear it out.

Would you say that the following "is where" okay for the same reason you drove in?

Dyslexia is where poeple have a learning disorder that impairs reading ability.

How do you tell if "is where ..." an adjective or an adverb clause?
Can you give more examples so that I can understand the role better?

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 BarbaraPA's reply was promoted to an answer.
Thanks Grammar Geek.