In the Classical era, grammar study was focused on a few idealized languages (Latin, Greek, Sanskrit); when people in the Early Modern decided to apply these rules to vernaculars, they ran into the problem that many modern languages don't follow such clean rules. The most common response was to try to insist that things that didn't look Latin shouldn't be considered "grammatical." Only quite recently did people begin working out how to describe the grammar of languages like English.

One consequence is that almost everything you're taught about English in school is completely and totally wrong. There's the completely bogus prescriptivisms that have no grounding (e.g., thou shall not end a sentence with a preposition). But even basic things like "what are the parts of speech" are pretty much wrong, being derived primarily from "this was what this Latin scholar said 2 millennia ago about a language that has a distant linguistic relationship to English."

It's also worth pointing out that the trend to conserve spelling and the written form probably obscures the underlying grammar as it changes. The French clitic pronouns seem rather more like recently-introduced inflections to the verbs rather than clitics, and the 's of English acts rather more like a particle than a genitive case marker.