Having a wide vocabulary will help someone write better essays (or anything in the writing category really), add fluency to speaking, comprehend what they read better, get better grades, and a whole lot more.
Vocabulary is the basis of language. Your vocabulary is simply the number of words that you know. The bigger your vocabulary, the better you will be able to communicate, and to understand. Reading is one good way to improve your vocabulary.
Here's a good website about 19th Century Sailors with a good vocabulary section.
To increase your vocabulary, try a dictionary.
look in the dictionary
Electronic Scrabble or crosswords are great for expanding a vocabulary.
flow and vocabulary
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It is always better to do practice than only reading. You will find some good quality SAT Vocabulary practice test and Free Sat Vocabulary Test at Skill-Guru
If you master vocabulary and syntax you can be a good writer.
I could answer your question if you be more specific and use correct grammar and vocabulary.
One way to know how large your vocabulary is in a foreign language is to count how many words you are confident of. Paying close attention to lessons is a good way to learn and frequent practice is needed to maintain or even add to a good vocabulary.
Body language, knowing what they are talking about and good vocabulary i guess...