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