But what actually are function words and what content words? To start with the latter, content words are all those words or word classes that, well, have content or an inherent meaning, such as nouns, verbs, adverbs and adjectives. Obviously, that makes the rest of all words function words, and among these we class articles (the, a/an), conjunctions (such as e.g. and / but / or / because etc.), prepositions (on / off / under etc.), or determiners and quantifiers (such as this / that, some / any etc.)

How many words do native speakers of English know at all?

According to an article whose link I'm providing below, the average native speaker of English knows around 100,000 words, or 10% of the entire vocabulary of the English language. This will, of course, vary significantly and depend heavily on the respective users' educational background, their jobs and relationships etc. What is striking though, is that although there are only 400 function words among the 100,000 words an average native speaker is said to know, these make up about 50% of the total word count this speaker uses on any given day!

If you'd like to read more about this topic, check out this article: 

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.561.9473&rep=rep1&type=pdf pages 347 et seq.

And on the importance of the word "the", we recommend:

https://www.bbc.co.uk/programmes/articles/1vhHx9Sv2bG7rVbZdWcSTcp/the-most-powerful-word-in-the-english-language