I love this video. Before going on about why, I would like to answer your question:
Yes, I find the numbers plausible. Based on observations on myself and others on this forum, and on our super challenge, it is not unreasonable to say that the 20000 pages (the original double challenge) could be the path to mastery, or at least a really good level. That is like 250000-300000 sentences (following your numbers. a random book I've just opened confirms it. But I've also got denser books, but the differences between individual books are not that important in that amount), that is twice or three times the middle level of comfortable reading that he mentions (100000). Let's take into account that in the living languages, you also add tons of listening to that number during the super challenge, so if we counted "sentences encountered" by both senses, we might get significantly more,perhaps up to 500000 sentences for one whole original double challenge. But not a million.
So, while I may personally think that mastery in a not too hard language may require less than the million (and I believe that talent and intelligence plays some part in that, but not the main one, sure), I am definitely convinced that you cannot read a million sentences and not be awesome. Therefore, it is a wonderful goal for a dedicated learner aiming for a high level.
I am tempted to take the 10000, 100000, 1000000 numbers as my goals next time
If there is any research proving those numbers, even better.
But I am tempted to post a link to this video next time I see someone complaining they are not fluent after two months of learning on the internet
What I love about the video: it explains and answers various questions:
-how can Latin be taught and learnt better. I wish I had known this back in high school, I could have become a proficient reader. If I remember correctly, our Latin teacher was switching from the traditional textbook to Lingua Latina the year after my class. What he talks about is much closer to my usual learning method. I believe strongly in the combination of grammar learnt without fear, a bit of translation, and tons and tons of input. But the disbalance in favour of grammar translation without the input was too strong even for my taste back then in the Latin classes.
-"The amount of reading is much more important than the quality." Please, repeat that every day to every language teacher with the snobbish attitude, who recomends either the most famous classics or nothing at all to their students. Let's also notice that the video actually talks about extensive reading in both the sense of reading a lot and not overanalysing everything with a dictionary ad nauseam. At least not in all the phases.
-a part of that attitude is a good learning curve, created by the appropriate reading choices. The author basically gives us a very clear overview on how to progress from the easiest step to the hardest one. I love that. Something similar is actually what I recommend to other people interested in extensive reading all the time (actually, I'd love to know what the author of the video thinks about the Latin Harry Potter and the Hobbit, whether he'd recommend them, and to which phase would he add them. I guess the easy non bilingual reads. Unless he is against translated modern works, that is possible). It is beautiful to see that this really is a universal principle, no matter what language you are learning. I find it fascinating, that in languages like Latin, it basically means travelling from now to older and older works. That is beautiful.
-I think this video is a good guide for all the learners interested in living languages primarily for reading. It doesn't patronise them like "all the good learners have conversations as a priority" (it cannot, Latin is dead), it very pragmatically shows that even the listening and speaking have their place in learning to read well as useful tools, not waste of time and torture.
-Another thing I really like is the idea of progressing from passive knowledge to speaking. That is something not rare on this forum, I'd say. But in the outside world, it is almost heresy. Speaking is being considered the universal holy grail. There is the goal of speaking from day one, and people obsess about langauge practice before they can even say anything, and people in classes learn mistakes due to being pushed to production they are not ready for. I am not saying the "speak asap" idea is universally wrong. But the other option, to prioritise comprehension first (and even for a long time, 1000000 sentences take a while) with production being a minority part of learning, is legitimate too.
Really, this video has so much to tell us about language learning in general, not only Latin! (And yes, it makes me desire to return to Latin right now!
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Thanks a lot for bringing it here!!!