I think you have exactly the right idea, when you ask if they are separate.
I am also very good at learning new words, and understanding their literal meanings. Our ability at this depends on how good our memories are, and how good we are at categorising things.
Putting the words together to get your meaning across, involves much more though. When you are putting words into a sentence, there are so many other things to consider besides the literal word meanings...
- How well do you know the other person?
- What mood you think the other person is in?
- What mood you are in?
- What knowledge do you and the other person share?
- Whether you like the person or not.
- Where are you? At work? In the street?
- Whether you are talking to them because you want to, or because you have to.
- How certain you are about what you are saying.
- ...and many more...
These parts of judging what to say, or what other people mean, can be difficult even for people with very big vocabularies. Those questions depend much more on knowing how people's minds work, and social etiquette. The rules for those are much, much less clear than the rules for what a word means and how you pronounce it.
Of course, that is putting it very simply. In reality, our vocabulary and our comprehension will influence each other a great deal. A bigger vocabulary means you have more choice to select the appropriate word from, and a good conversation might teach you some new words.
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