If you are thinking of a career in the US military, you have to study intensively for your upcoming professional military skills or for the ASVAB exam.
One of the topics handled by the army ASVAB vocabulary practice test is Working Knowledge (WK). In this section, you need to know many vocabulary words. Most words that you may or may not know. Entering this part blindly is not ideal because you might not know the answer if you don't recognize the word.
However, the good news is that you can easily master this part by learning the vocabulary. You can buy a set of flashcards with a dictionary or a card specifically developed for ASVAB or a set of SAT flashcards.
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Although it is not possible to memorize hundreds and thousands of words in one frame, you can slowly but surely build your vocabulary by studying 10 or more flashcards every day. When you study index cards, you learn the word. Learn the definitions and then practice using words in sentences.
The last tip for improving your vocabulary is reading. Set goals for reading at least one novel every week until your exam. If you only learn one dictionary, you will increase your knowledge of words and definitions.
However, by reading, you can apply these words and see them in context and action. This allows you to understand English much better and hence gives you a better chance of getting a high score in your ASVAB exam dictionary area.