To learn a language you have to learn the words used in that language. You have to learn many words, how they are used, when they are used and which words are used together. That is all you need to learn. If you learn many words and how they are used, in a natural way, the grammar will be learned, also in a natural way.
If you just want to have a superficial social competence in a new language you may get by with a few words, the most frequently used words for example. If you want to be fluent and to be able to cope with most situations that matter to you, you need to learn a lot of words. This includes the less frequently used words.
The less frequently used words are harder to learn precisely because they appear less frequently. You need an efficient method to learn them. Chances are that you will learn words that matter to you and which you need. Then, when you are tested you should only be tested on the words you claim to know.
That is why we developed the “language learning engine” at The Linguist. Our statistics on learned words and phrases and known words etc. are designed to give you a clear measurement on your progress towards your goals. The measurement of the number of words you know, once verified, is a simple and effective indication of your ability in a language.