The 80–20 rule is a principle that was devised almost one hundred years ago but has been popularized fairly recently by many personal development writers and bloggers. It states that in most areas in life, roughly 20% of the input is responsible for 80% of the output.
If you write an email, 20% of the time you spend writing it creates 80% of that email. 80% of an email means that it is not perfect (this would be 100%), but it is probably understandable enough to send. So if you spent ten minutes writing an email, the first two minutes were spent getting 80% of the email done and the remaining eight minutes were spent trying to perfect the email, when it was already good enough to send after two minutes. In language learning and teaching, it is the same. It means you should learn and teach the most important things to students, not the obscure rules and exceptions that students will hardly ever have to deal with.
It states that in most areas in life, roughly 20% of the input is responsible for 80% of the output.
The same rule can be applied to language learning and teaching. It is commonly believed that 50% of all conversation in English is made up of only 100 basic words, and that 80% of conversation only uses about 3,000 words. Similar figures have been attributed to most other languages as well.
This means that if you know the most basic and commonly used 3,000 words in a language, you should be able to understand 80% of a conversation. We can assume that, in most cases, context would be enough to understand the remaining 20%.
Recognizing 3,000 words in a foreign language and being able to use them are two different things, though. By bearing this rule in mind, you will know which grammar points, tenses or sentence structures to focus on during your next lesson, whether you are a student or a teacher.