A great way to improve your English vocabulary is by being an attentive reader. Without a doubt, you encounter unfamiliar words as you read; learn how to use these moments as opportunities to improve your diction. This post will focus on how to determine meaning from context, keep track of new vocabulary, and incorporate new words into your own working vocabulary.
To help you make educated guesses about new words, study common root words, prefixes, and suffixes: Etymologies Handout
And, a separate set of specialized Academic Word Lists, by discipline.
And, finally, a list of games and worksheets for further practice.