Guide

Introduction

I'm a very strong advocate of comprehensible input, but there are a lot of people saying "comprehensible input" when they just mean "input," as if it helps you very much to listen to a lot of language that's too fast or complex for you to understand anything.

What do I mean by "Comprehensible Input"?

Comprehensible inputs is key. I don't think anybody that has gotten to "fluency" in any language without comprehensible inputs in one way or another. Personally, I didn't realize this until later in my studies but as I got more advanced, the more I realized that not just inputs but comprehensible inputs is what is important. Simply put, incomprehensible inputs is better than nothing but it's practically useless. Comprehensible inputs will teach you language nuisances, new vocabs, solidified previously learned concepts, language intuitions(grammar), .etc.

It has a very important part that people gloss over. In order to be useful, the input must be over 90% comprehensible to the one receiving the input and the learner generally has to receive input on the same word/grammar nearly a dozen times for it to actually embed itself. It's not comprehensible input unless you can understand it. That is what "comprehensible" means.

If the input contains too many things the learner cannot figure out, it is no longer comprehensible. If you're reading or listening to something and only understand a few words, it's not comprehensible. If you hear a word you don't understand and you cannot figure out what it could mean, it's not comprehensible. (Using a dictionary =/= comprehensible input.).

I'm not huge on classroom learning but thinking back on it, my teacher speaking a dumbed down version of the language helped a lot in solidifying. For a long time, I used Anki as my main source of studying. But all the thousands of hours of Anki studying didn't really solidify until much later; it came from hundreds, if not thousands of exposures with contexts from comprehensible native resources.

Learning languages like a baby?

Comprehensible input for a brand new learner usually doesn't look like watching your favorite movies without subtitles because your language level isn't ready for that. And it's not like how "babies learn" because you're not a baby and you'll never again have the chance to learn like a baby learns.

You don't really wanna learn like a baby. At least, not in the beginning. As a beginner, an hour spent reading a textbook will give you about a hundred times more bang for your buck than an hour spent watching a TV-show. A few months in textbooks and other learning materials will slowly stop giving those massive returns in investment and immersion will gradually take its place as the main source of learning, but the nature of the beast is that the more of the language you know the easier it gets to learn even more. The corollary to this is that the less of a language you know the harder it will be to learn more. And since in the beginning you know nothing at all, you gotta do at least some studying. A textbook will straight-up tell you "This word means that thing. That grammar point means this thing". In the beginning you will need that.

Research on both babies and adult second language learners suggests that we don't learn language just by hearing it. (Hearing babies of deaf parents don't learn language from TV!). We need to hear it at a level that is deliberately slowed down and simplified so that we can understand it. Mostly, this means learning from teachers who are trained in CI techniques, rather than self-study; most of the work I've seen on comprehensible input is specifically classroom-based. But CI proponents also advocate a lot of reading, using resources like the earliest levels of these graded readers.

Start comsuming comprehensible input.

It's quite difficult to do comprehensible input as self-study if there aren't good materials available, but it was possible to put something together between textbooks, graded readers, and podcasts intended for second-language learners. I think that if you're not in a situation where you can take classes from someone who uses CI techniques, then you kind of have to use more traditional study methods (mixed with as much comprehensible input as possible) until you're at a level where you can understand native content at a decent level.

Reading a lot of stuff way above your level is not what proponents of CI suggest you do because it can be very slow and discouraging - but some people are carried through by sheer passion and ambition, like I was. It definitely helped me relative to the other people in my class who weren't doing as much out-of-class reading as I was.

In my opinion, comprehensible input is great but it's not the cure to learning a language. It's part of the prescription, joined with many other things. I'm not sure how comprehensible input could work for someone trying to self-study unless you're following a plan made by someone else. Otherwise it'll be really hard to find material that is comprehensible to you. Most media made for native speaker isn't truly comprehensible to learners until they've entered intermediate or advanced level.

Immersion

If you're already at a high-intermediate level, then watching TV/movies without subtitles can be comprehensible input, but if you're a beginner, it's not. For comprehensible input, you're looking for content you can understand 95%-98% of. (You will get SOME comprehensible input if you understand less than that, but if you can understand very little of what you hear, it's not comprehensible input anymore). Not using subtitles has nothing to do with comprehensible input. Comprehensible input proponents generally encourage people to use subtitles (but more importantly, you should find content that's so easy you don't need subtitles).

Of course, immersion is something you do in your free time, so you can do it even in the beginning, after you're done with your daily hour of studying. And to avoid forgetting what you've learned, use a memory aid program like Anki. Stick sentences in Japanese on the front card, what they mean on the back card.

(By the way, read Principles and Practice in Second Language Acquisition by Stephen Krashen; it's a free book by the guy who literally wrote the book on comprehensible input).