Latin has an inflected grammar, in which words change their form to indicate the role they're playing in a sentence. English has a little bit of inflection; Latin has a lot. For example, in English, these are all the possible forms of a verb: show, shows, showed, shown, showing. Most Latin verbs have about 150 different forms. These indicate how the verb fits into the sentence, which noun it agrees with, and other things.
English has a possessive case for nearly all nouns, indicated by -’s, and distinguishes two other cases only in a few pronouns: I/me, he/him, she/her, they/them, and (sometimes) who/whom. Latin has five cases (or six, or seven, depending on how you count them), and they apply to all nouns. For example, in the Latin sentence Marcus Quintum pulsat, which means “Mark hit Quintus”, the -us ending indicates that Marcus is doing the hitting, and the -um ending indicates that Quintus is the person getting hit. The word order is very flexible, because the cases indicate how the sentence fits together. You could say Quintum pulsat Marcus and it would mean the same thing.
In Latin, all the grammatical distinctions and categories are explicitly indicated by the inflections. The meaning of most sentences results from combining the meaning of each individual word according to the grammatical rules (known as “the principle of compositionality”). It’s easy to see why grammarians would like to use the concepts of Latin grammar to explain the grammar of all languages. Latin has rules, which are mostly clear and obvious.
But English grammar works very differently from Latin. English grammar is more of a patchwork of phrases, with a few inflections to help out (like -ed, -s, and -ing). Many phrases have unique rules for how to use them, like “too adjective to verb”. Grammatical distinctions are often indicated by ambiguous auxiliary verbs like will, would, and should, whose meaning varies a great deal depending on the phrase or the context. Here is a list of verbs, some of which can be followed by a gerund, some of which can be followed by an infinitive, and some of which can be followed by either. There is no rhyme or reason to the list.
Unlike Latin grammar, which the Ancient Romans had written about and explained in detail just from noticing when they use each inflection, English grammar is quite hard to figure out and explain. Even today, linguists are still debating over what the grammatical categories are. For example, sometimes there seems to be no way to tell which (Latin-like) grammatical category is denoted by -ing, as in this question. Probably the most complete attempt to formulate English grammar as a set of rules is this book, which is 1,800 pages long and costs US$250.00!
The effect on education
For a couple hundred years, many educators thought you could understand English grammar by learning Latin grammar. It was thought that English grammar was doing in an “abstract” or “invisible” way what Latin grammar does “concretely” or “visibly” with inflections. So, learning Latin grammar seemed like an easier way to learn the grammatical concepts of English. For example, Dorothy Sayers (1893–1957) said “To embark on any complex English construction without the Latin Grammar is like trying to find one's way across country without map or signposts.” The idea that Latin and English grammar do not just represent the same things in superficially different ways, but work in profoundly different ways, so that the “map” you learn from one language misleads you in the other language, did not seem to occur to many people. The assumption that, deep down, English and Latin grammar work the same way, is much less popular now than it used to be, but a few people still believe it. (This assumption might even still influence linguists of the type who try to work on a “universal grammar”.)
Consequently, some of what most Americans are commonly taught about English grammar is wrong or confused, because American schoolteachers’ grammar concepts are largely borrowed from Latin.
For example, English verbs have an -ing form, which doesn’t neatly correspond to anything in Latin. Some people are taught that the word running in “I am running” is a present participle, and others are taught that it’s a gerund. Present participle and gerund are concepts from Latin grammar. However, in the English sentence “I am running”, neither of the Latin grammatical concepts fits well. Yet, schoolteachers teach the Latin grammatical terminology as if it applied to English grammar. Mostly this causes confusion. The schoolteachers themselves probably don’t even know they’re doing this.
Two famous errors, “taught” up until about twenty years ago, were that it’s wrong to end a sentence with a preposition, as in “Jane picked the cat up”, and wrong to “split” an infinitive, as in “to boldly go”. These are natural, grammatical constructions in English. However, under the influence of Latin grammar, many educators thought they were ungrammatical. The problem is that phrases like pick up and to go are two-word units of meaning in English grammar unlike anything in Latin grammar.
In Latin, a preposition always introduces a prepositional phrase, which includes a noun. If you say in in Latin, you always follow that by saying in what. For example, Marcus Quintum in via pulsat, “Mark hit Quintus in the road.” In Latin, it really doesn’t make any sense to end a sentence with a preposition. No one would do that, just as in English, no one would end a sentence with an article.
And Latin has nothing corresponding to the to-infinitive of English. It’s not possible to split an infinitive in Latin even if you wanted to. Many people learning English as a foreign language have been misled by being told that to verb is how you make an infinitive in English, by false analogy with Latin infinitives. Hence “
I go for to buy cigarettes.”
To this day, most Americans do not know what phrasal verbs are, even though they use them every day. And many people learning English as a foreign language are never taught about phrasal verbs, because phrasal verbs don’t correspond to anything in Latin grammar. Lack of a name or even a concept for phrasal constructions causes a lot of confusion for people whose native language doesn’t have them.