Crossword puzzles are a quick and easy way to practice your vocabulary skills and this site has a large number of puzzles ranging from “very easy”, which features the most commonly used words, to “difficult”. Why not give one a try?
Unlike Japanese, English doesn't have radically different dialects (方言). The average American can understand the average Brit with little difficulty. An Irishman can converse easily with someone from New Zealand.
There are some words, however, that are different.
List of 100 Most Common Words
List of 1000 Most Common Words
List of 3000 Most Common Words
1. Read, read, and read.
The easiest way to do this is to read about things you like in English. If, for example, you like gardening, then read about gardening in English. You can find all kinds of websites, magazines and books on that topic.
Another way is to read what are called Graded Readers. These are often simplified versions of famous novels. Oxford Bookworms have books in five levels 1~5. Penguin, too, has a similar series. There are also graded readers for children. The Step Into Reading series is recommended.