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Thanks to your reply to my last question (He has (a) good knowledge of English grammar.)
I understood that "a" is obligatory in this case. However,..

I have been learning each question and answer thoroughly at the Grammar Help section for a long time and once I came across the following post:

(a) Mary showed good understanding of the musical concepts.

(b) Mary showed a good understanding of the musical concepts.

Are both of the following sentences grammatically correct?

Your response: Both are possible, but the second is slightly more natural.

My questions :
1) Are a good knowledge and (a) good understanding labelled as the same category?
2) Why does one take an indefinite article *in an obligatory way whereas the other does so optionally?

Many thanks.


Source: http://www.englishclub.com/esl-forums/v ... erstanding

P.S. I am not certain whether the adverb obligatorily is appropriate or not

I'm afraid that it is determined case by case, depending on the noun.

A good learners' dictionary should help you decide whether the article in such cases is necessary.