There is such a thing in English that when a mass noun combined with an adjective it takes the indefinite article. For example:
He had a great knowledge of English.
But I have noticed that is not always the case. For example: I have observed that with the word "fluency" and other adjective the is not followed, for instance:
She showed great fluency in English.
Would it be correct to say "She showed a great fluency in English"? Tell me please if there is any additional rule to that.
Best Answer
There is a rule in Michael Swan's "Practical English Usage" (section 149.4):
Unfortunately this book does't show a list of such uncountable nouns which can/cannot be used with "a". But the Ngram Viewer shows that "fluency" is not used with "a". See the results of searching:
So, it would be better to say "She showed great fluency in English".