Such difficult an exercise was it that no one could do it.

[Nonverbis]

Upstream Proficiency by Virginia Evans and Jenny Dooley.

The task was: The following sentences contain mistakes. Find them and correct them.

Such difficult an exercise was it that no one could do it.

My answer was: Such a difficult.

The answer key:



Well, this answer key looks strange to me.

Could you help me with two questions:
1. Is my variant correct?
2. Is the answer key wrong?

 

[5jj]

The correct sentence is; So difficult an exercise was it that no one could do it.

 

[Nonverbis]

Could you explain why?

The dictionary gives this example: It's such a beautiful day!

such determiner - Definition, pictures, pronunciation and usage notes | Oxford Advanced Learner's Dictionary at OxfordLearnersDictionaries.com

Definition of such determiner in Oxford Advanced Learner's Dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more.

www.oxfordlearnersdictionaries.com

Why can't it be "Such a difficult exercise"?

 

[Rover_KE]

You'd need to rephrase the sentence:

It was such a difficult exercise that no one could do it.

 

[Glizdka]

Such connects to nouns, only. So connects to not-nouns.

The position of a is crucial. If an indefinite article appears before the adjective, the adjective is part of the noun, and they both function as "one entity". You need to use such.

If it appears after the adjective, the adjective and the noun are "separate entities", and you're no longer connecting to the noun; you're connecting to the adjective only. You need to use so.

"So difficult an exercise."
"Such a difficult exercise."

The latter is much more common. Exams, however, tend to check rare patterns, such as this one.

"Such difficult an exercise" doesn't work because "Such difficult" doesn't work.