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Hi there,

Fellow polyglot here, I've mostly studied Spanish, Russian, and Vietnamese.

Most people really struggle with language learning so I'm glad to see a post like this here. I'd like to add some notes I've learned in my own language-learning journey.

1. Textbooks — Textbooks are useful but people misuse them in really self-sabotage-y ways. I recommend spending less than 5% of your time doing textbook-related things. This is because they're very boring and kill motivation, and because they mostly build declarative/conceptual understanding rather than procedural knowledge.
2. Movies/cartoons/songs — My school classes weren't set up to teach listening comprehension, so I turned to kids' cartoons for practice. But they didn't actually help because I couldn't follow what anyone was saying. So then I started writing transcripts of everything they said. I would pause and go back over the sentences I didn't understand, to the point where it took me like 12 hours to watch a single 20-minute episode. Actually like 99% of my study-time consisted of transcribing cartoons and drilling flash cards. This worked unreasonably well for me, but I haven't seen anyone else do this. I think most other successful language learners mainly read books in their target language.
3. Vocabulary — The biggest bottleneck in language learning, by a wide margin. Sure, grammar is fun and interesting, But Vocab is absolutely key and drilling flash cards is ALWAYS a great idea. In my experience, 15 minutes of drilling corresponds to 1 learned word, where "learned" means that I'll remember that word 6 months later. When you're just starting out with a new language, and aren't familiar with its sounds and relative frequencies of morphemes, this rate can be a lot slower, though. 
4. Sleep — My learning ability degrades a lot when I'm tired. Once I started keeping a consistent study routine this became really obvious really quickly. So I make sure to sleep enough when I'm doing mental work.
5. Listening to Music — Another thing I noticed with my study routine was that listening to music degraded my efficiency in studying. I still just continued listening to music anyway because it was more fun. I mean I honestly don't think I would have had the patience to study flashcards for like an hour every day without the music, so it helped in that sense, but just comparing the days where I studied with music, vs without, I finished my study routine 20-30% faster on silent days.
6. Grammar — Grammar is by far my favorite part of learning languages. Firstly, most people don't like grammar and they don't learn it perfectly. All of my Spanish teachers got the Subjunctive Mood wrong, even at the college level, so be careful about what people say about grammar. The very basics—subject, object, verb, adjective— You can trust that, but beyond that be skeptical! Secondly, you can get really far on raw intuition just by getting lots of exposure to the target language. So I'd recommend not worrying too much about grammar and just focusing on taking in the language. I mean, if you love grammar like me you should learn it, but don't worry if you're not like me!
7. Classes — I think learning in a classroom setting is extremely inefficient. The main advantage of classes is that you HAVE to go to them so they force you to learn, but getting a private tutor is much much better. If you're intrinsically motivated enough, the tutor isn't necessary, but they always help a lot. I think for a private tutor, at least 3 sessions per week is necessary. 1 session per week isn't enough.
8. Study Abroad — I did a 1-year study abroad and, well it's hard to evaluate, actually. I used Spanish a lot there, but I also retreated back to English-Speaking-Internet during my down time, and I stopped doing my normal Study regime, so I don't think I got all the value out of it that I could have. But using Spanish all day definitely helped me with holding conversations in Spanish, which is nice. I don't really consider myself fluent, but I do alright in small groups and can talk about most topics, so I'd say overall it was a success.
9. One of the problems that I haven't really solved is getting Spanish as an actual part of my daily life. When I used to study Spanish it was always scheduled time that I specifically dedicated to practicing Spanish. It was always effortful, like fighting against a current. But ESL people get a lot of automatic practice in English since it's the Lingua Franca of the internet, and all the most interesting parts of the internet are in English. I mean, how cool an experience is it, to grow up in like Portugal, learn English through the internet, and find all the cool places like LessWrong, and those hour-long speedrunner video essays, and the special-interest chatrooms in English? Isn't that such an awesome way to learn a foreign language? I don't know, I guess I'm just kind of jealous. This is just a rant though, not useful advice. Don't beat yourself up though if you're not as fluent in your target language as your ESL friend is in English.

I'm curious if your experience lines up with mine? I think fundamentally, the main reason I'm successful at language learning is just because I love it so much and can do it for long periods of time without exerting as much willpower, but I'd love to know if my advice is useful to others.

Domain: Linguistics

Link: [The Art of Language Invention, Episode 25: Ghost Segments]

Person: David J. Peterson

Background: Writer of many 'conlangs' (artificial languages) such as Dothraki from Game of Thrones

Why: 30+ part video-series about conlangs. In theory, it's meant as a resource to guide your own creation of a language. But it's also just a really good resource for absorbing how a linguist thinks about language. He talks about sounds/words/grammar, how they change over time, and what mechanisms are involved in that. IIRC he doesn't use very many technical terms but his depth of knowledge is very obviously deep. In my own case, this video series, and the accompanying book of the same name were instrumental in my linguistics journey.