You know how disappointed it can be if you are not able to communicate with the (native) when you're traveling, and you might think learning a new language is a tough task.
But fear not! We figured out (exact) how many words you need to learn to speak a language well. A group of linguists(语言学家) have created a test (see) how many words you know in English. There are 200,000+ words in the English language, but the test only (include)50 of them. You may not understand all of the50 words. The linguists believe that if you multiply (乘) number of words out of50 that you understand by500, you'll almost know your English vocabulary.
A professor has found that native speakers of a language know 15,000 to 20.000 lemmas (词目). A lemma (make) up of a root word (say: walk) and all of (it) inflections(屈折形式)(walked, walking, walker, etc.). But if you learn a new language, you can never understand 15,000 lemmas.
He discourages people from learning all of the words in a language. There are 800 to 1,000 lemmas are used most frequently in the language. He advises people to focus these lemmas. With these lemmas, people can understand 75% of the language. And they will be (confidence) enough to start a conversation, order at a restaurant, and deal with other daily communication.