And though I live in France, I’m not immersed enough. I use French for work, but I speak lots of English too, including with my kids and husband. I don’t have an “école horizontale” — a romantic partner with whom I speak only French.

I’ve tried to compensate by periodically taking French courses. And most mornings, I circle unknown words in Le Monde, then transfer them to sticky notes above my desk. But I recently discovered three notes reminding me that “ras-le-bol” means “fed up.”

“Nothing seems to work as well as just speaking the language all the time,” Dr. Hartshorne said.

You can learn basic grammar and vocabulary at any age. That explains my “good enough” French. But there’s also an enormous amount of low-frequency words and syntax that even native speakers might encounter only once a year. Knowing any one of these “occasional” words or phrasings isn’t essential. But in every context — a book, an article or conversation — there will probably be several. They’re part of what gives native speech its richness.

In other words, no matter how many sentences I memorize or words I circle, there will always be more. “You can get pretty good pretty quickly, but getting really, really good takes forever,” Dr. Hartshorne explained.

And your peak level might not last. I used to interview people in Portuguese; now the language merely sounds familiar. Most of what remains from three years of Japanese is a haiku I learned for extra credit in high school.

Confidence matters too. It doesn’t help that with French, I’m studying a language that’s considered such a treasure that it’s presided over by a group known as “the immortals.”

Dr. Hartshorne also points out that native speakers have exceptional precision. Even someone with 99 percent grammatical accuracy sounds foreign. He guesses that I have about 90 percent accuracy, which shouldn’t feel like failure. “Imagine if you decided you were going to pick up golf in your 30s, and you got to the point where you could keep up in a game with professional players. You’d think that’s actually really good. But for some reason, just being able to keep up in language feels not as impressive.”