Thanks, Marius.
I will keep this rule in mind. What I am puzzled by, however, is the fact that either of the examples you gave me doesn't refer exactly to the sentence I gave.
In your first example, we deal with proper names, no articles there, and in the second one, with one noun in plural and the other in singular.
If I kept this rule in my sentence, I should write "He doesn't even know what an adjective and adverb are," which sounds strange to me because of the indefinite article.
Of course, I will take GG advice and write "adjectives" and "adverbs", but that doesn't clarify my doubts I had in case of the original sentence
![Sad [:(]](/emoticons/emotion-6.gif)
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Best regards
![Smile [:)]](/emoticons/emotion-1.gif)
Magda