I find that many learners have an obsession with slang and idioms, as if trotting out some very col oquial expression is going to upgrade how they sound in a language. In fact it is the opposite. Idioms are difficult to use correctly. They can easily sound very strange coming from someone who obviously does not have a good sense of the language.
Yes, occasionally you hear idioms or slang and do not understand. But, in my experience, there are always situations where you do not understand. It is best to focus on the most important words, what they mean and how they are used. Certainly, it is best for a non-native speaker to stick to standard speech.
Some teachers even try to teach the "real language." Then you hear non-native speakers saying things like "I gonna" "You wanna" etc. I just cringe. You will learn al the idioms you need just by exposing yourself to the language.
Nation defines comprehensible input as content with only one in fifty unknown words. That is 2%! I think this number is too low to be realistic.
First of al , when you start a new language, and for a long time thereafter, it is difficult to find any content, and in particular interesting content, that meets this definition of comprehensible. For a long time most content wil have a much higher number of unknown words, unless you read painful y childish content, which is unlikely to motivate a learner.
Second, the main job in language learning, in my view, is to acquire words and phrases. If you have to read fifty words, for every new word you learn, you wil have to read an awful lot in order to acquire a fluency level vocabulary, which I consider to be over ten thousand words.
I prefer the term meaningful input, where the interest of the learner is taken into consideration. I am not interested in reading children's stories with a lot of easy words. I prefer to struggle through the authentic version of some novel, conversation or news report, as soon as I am able, even though there may be 40 or 50% unknown words. Having the chance to listen, as wel as read, helps. Having access to an online dictionary helps. Using LingQ helps.
Using LingQ for Russian I have seen the percentage of new words come down from 40-50% to a level of roughly 20% for the novels I am learning. Some podcasts are now down to 10%.
Using LingQ, that level is quite comfortable. Other LingQ members have told me the same thing.
So I consider that an unknown word rate of around 10-20% is a reasonable balance between ensuring that the content is "comprehensible‖, on the one hand, and that new words are being acquired, on the other hand. It is often more important that the content be of interes t to the learner. This assumes that it is a learning assisted form of input, such as reading and listening at LingQ. Reading a book, unassisted, is another matter.
I consider this range of difficulty (10-20% unknown words) to be an ideal "vocabulary acquisition zone", just like the "fat-burning" or "aerobic" zones that we can achieve when running on a tread mil . I think it is an ideal level of difficulty for a language learner.
Many language learning sites offer a "word of the day" service, which is amusing for learners, even though I do not find it al that useful for learning a language. I mean you need thousands, or even tens of thousands of words. So, at one word a day, it would take a long time to learn a language. What is more, getting words devoid of any meaningful context, I mean meaningful to the learner, is quite useless, so most of these words wil be quickly forgotten.
My wife likes to do crossword puzzles. I sometimes help her, and this morning I discovered that on the same page as the crossword puzzle, there is a Word of the Day item, sponsored by CanWest Canspel and the Post Office! Today's word was Niebelung! That's right, Niebelung, definition "a member of a Scandinavian race of dwarfs."
My wife keeps her old crosswords so I looked up some of the recent Words of the Day, which I list at random below. Either these words are known to the reader, or they are quite useless and wil not be learned. If anyone comes across an unknown word, it is easy enough to search the Web or a dictionary for a definition and examples of usage. In any case, these words have nothing to do with the effort to improve literacy among low literacy people, i.e. those people who have trouble reading.
Niebelung
Weir
Riparian
Osprey
Lunula
Paleontologist
Perihelion
Febrile
Exequies
Fusilier
Ochre
Necromancy
Discrete
Rapscal ion