Hello,
Many people find sentence equivalence and text completion in GRE as most difficult part. The words can be learned in simple way as well. Going in Google and searching for prefix, suffix, and root words is one main part you need to learn. Once you know most of them, then check the sentence equivalence and text completion, look at the blank and come up with your own word. Your own word will first help you decide whether it is positive or negative word, then prefixes like un, in, or dis, can be eliminated easily if the word you thought is positive. This way by the end of the time you will come up with your right answers.