I'll use myself as an example - my Spanish vocabulary is great! I know hundreds of individual words - nouns, adjectives, infinitives. prepositions. When I take a vocab test online, I pass with flying colours. Can I speak Spanish well? No, I can't. My tenses are all over the place. My verb endings are rubbish. My singular/plural usage is appalling. I use the wrong preposition for the context all the time. I can make myself understood but I do not, by any means, speak good Spanish despite knowing a lot of words.