This book is basically "How to use a spaced repetition system to learn a language" plus several great tips. It is best used when you’re just starting out with a particular language (damn I wish I read it before I started learning Swedish), but also gives pointers and adjustments to people on intermediate level.
A spaced repetition system is a proven way to remember as much as possible in as little time as possible (and also the only reason I passed the driving theory test). If you already use one (e.g. implemented in Anki), you can safely skip the parts of the book where the system is explained.
Other tips include:
start with mimicking the pronunciation (that’s how secret agents do it ;) )
train listening to 'minimal pairs' until you can actually tell them apart (e.g. ship/sheep; if you can hear the difference, you can try speaking it too)
don’t translate between languages, learn to speak the target one (e.g. by putting a picture of a cat on your flash card instead of the word 'cat' in your native language)
add imagery, associations, personal context/memories wherever possible for easier memorization (there’s also practical advice on how to do it for something more abstract than cats).
There are some additional tool recommendations, and some of those are not available anymore. The author’s website has a page for updates of this sort; pro-tip: for the dead lang-8 (where native speakers used to correct your writing for free) the new alternative not yet mentioned on the site is https://langcorrect.com.
If you squint, you can also apply a lot of this to any other kind of learning, so that’s a plus. Speaking of applying stuff, technically this book is good for any language on Earth, but depending on the popularity of the particular one you’re learning it might require less or more (or way more) effort.
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