Hi,
Consider this sentence.
He has a very good knowledge.
What is the function of the aritcle 'a' in the sentence?
( I know that the articles can function as adjectives but never as adverbs..Well, the case here is different, isn't it?....Can an indefinite article modify an uncountable noun? )
Article use can be difficult to learn, and to explain, as you perhaps are aware. Let me offer a few comments.
We don't say 'He has a knowledge'
We do say 'He has a knowledge of French, but he does not have a knowledge of Russian or a knowledge of Chinese.' In this context, we are thinking of and speaking of several kinds of knowledge.
We don't do this with the plural , eg we don't say He has knowledges of Spanish and Japanese.
We wouldn't normally say the sentence you are offering as an example, because it does not specify a type of knowledge. However, we could readily say He has a very good knowledge of English.
Best wishes, Clive