These are the most common difficult words found in SAT and GRE exams, culled from dozens of exams.
You can study and practice them in 50 groups of 10 words each or practice randomly.
The first 30 are prefixes, suffixes, and roots.
To be prepared for the exams, you should learn 20-30 new words a day and make 3x5 cards for ones that you have difficulty with, and review all your cards regularly.