how would you breakdown these words using root meanings, pretentious, evanescent, avarice, and astute.

Answer 1

Look them up in the dictionary; it will give you the root of the word.

Example from the on line Merriam Webster Dictionary:

pre·ten·tious
Pronunciation: \pri-ˈten(t)-shəs\
Function: adjective
Etymology: French prétentieux, from prétention pretension, from Medieval Latin pretention-, pretentio, from Latin praetendere
Date: 1832

Answer 2

GuruBlue is right. Any good dictionary will help you with these types of things.

Another good place is an etymological dictionary, such as http://www.etymonline.com/