> Native speakers don’t start studying grammar until they have had 10+ years of full-time experience with the language.

Untrue; formal grammar instruction begins not later than first grade in many curricula, which is age 6-7, which would require using the language several years before birth to reach 10+ years prior use. Native speakers begin studying grammar about as soon as they have the intellectual capacity to comprehend the concepts associated with grammar.

There is little if any formal instruction in grammar in reasonable primary schools. All else equal, students who attend primary schools that don’t teach grammar at all end up speaking and writing just as well as students who attend primary schools that try to teach grammar.

When primary schools try to teach grammar it is boring, stressful, and generally unhelpful to the students.

The dominant factor affecting students’ reading comprehension and writing ability is how much time they spend listening and reading, especially to material which is at an appropriate level to slightly stretch their abilities.

If schools want to spend a relatively small amount of time formally teaching grammar to 12–17 year old students there’s nothing inherently wrong with that, but it’s mostly only useful insofar as it attaches names to concepts so that students can have conversations with each-other about what makes communication effective of ineffective, or more explicitly discuss their existing subconscious grammatical knowledge. Formal instruction in grammar (or other kinds of formal analysis) is still no substitute for practice listening and speaking and reading and writing (ideally with effective feedback), which should be the main focus of language arts instruction.

> There is little if any formal instruction in grammar in reasonable primary schools

Maybe we need to bring back grammar school.

> Although the term scolae grammaticales was not widely used until the 14th century, the earliest such schools appeared from the sixth century, e.g. the King's School, Canterbury (founded 597) and the King's School, Rochester (604). The schools were attached to cathedrals and monasteries, teaching Latin – the language of the church – to future priests and monks. Other subjects required for religious work were occasionally added, including music and verse (for liturgy), astronomy and mathematics (for the church calendar) and law (for administration).

I am not knowledgeable enough about the topic to say how efficient or helpful medieval Latin schools were at teaching Latin as a second language, but the idea of “grammar” as part of the “trivium” (alongside logic and rhetoric) meant something substantially different than the modern usage of the word.

I can remember being taught about grammar in first grade. And getting it wrong at first.

But I don't follow the basic assumption that the way native speakers become fluent is particularly effective. No matter how much explicit instruction helps, native speakers couldn't learn basic fluency that way, because there's no way to deliver explicit instruction to a baby.

Agreed, but learning grammar before you can even understand the examples used to illustrate a rule is putting the cart before the horse.

On the other hand, once you know a few specific instances, it's helpful to explicitly point out that they're governed by the same rule and give an abstract statement of how the rule works. That way you can check yourself without relying on a teacher to point out your mistakes.

Providing a few specific instances and then talking about the general rules is typically how grammar is taught when acquiring second languages.

Being shown a few examples is not the same as understanding them. If the first time you see an example of the rule is during the explanation of the rule, you have to deal with too much new information at once. A good explanation should refer mostly to information the learner is already familiar with.

The first time you see an example of the rule, there's no need to explain the rule yet. You can just memorize the example. The same is probably true of the second and third example. Explaining the rule only makes sense once remembering the explanation becomes easier than remembering the set of examples you need to know.

Good textbooks already do exactly this. Show a few examples of particular sentence structure, using vocabulary and syntax at the level of the learner, and then briefly draw attention to a general rule at the end.

I've heard toddlers learn about 7 words a week.

When I'm into a new language I can learn 20-50 words a day, sustainably. Short term more.

If you define “toddler” as age 1–1.5 then sure.

By the time a child is 2.5 or 3 years old they can be learning dozens of words per day.

The most efficient way to teach new words to kids is to spend as much time as possible reading books with them.

Grammar for learning a second language is helpful. Helps you to understand how to map what you want to say, and what you heard/read.

That said, brute force practice work reeeeally well for language. And naturally people will build a kind of internal grammar anyway. But knowing what is what can help with that too.

> Native speakers don’t start studying grammar until they have had 10+ years of full-time experience with the language.

That's by necessity because you can't explain perfect tense to a newborn.