Knowing one Greek and Latin root means that you know many words associated with that root. Score one for efficiency.
If you understand that any time you see the root, theo-, you're going to be dealing with "god" in some form, you'd know that words like theocracy, theology, atheist, polytheistic, and others all have something to do with a deity even if you've never seen or heard those words before. Knowing one root can multiply your vocabulary in an instant.
Knowing one suffix, or the word ending can often give you the part of speech of a word, which can help you know how to use it in a sentence.
Definition: a person who...
A word that ends in -ist will usually be a noun and will refer to a person's job, ability, or tendencies. For instance, a cyclist is a person who cycles. A guitarist is a person who plays the guitar. A typist is a person who types. A somnambulist is a person who sleepwalks (som = sleep, ambul = walk, ist = a person who).
Knowing the prefix, or the word beginning can help you understand part of the word, which is really helpful on a multiple choice vocabulary test.
Suffix: a-, an-
Definition: without, not
Atypical means not typical or unusual. Amoral means without morals. Anaerobic means without air or oxygen. If you understand a prefix, you’ll have a better time guessing the definition of a word you may not have seen before.
Every major standardized test requires you to understand more difficult vocabulary than you've seen or used before. No, you won’t have to write the definition of a word down or select a synonym from a list anymore, but you will have to know the complex vocabulary, anyway.
Take, for instance, the word incongruous. Let's say it appears in the Redesigned PSAT Writing and Language Test. You have no idea what it means and it's in the question. Your correct answer relies on your vocabulary comprehension. If you remember that the Latin root “congruence” means “to come together” and the prefix in- negates what’s behind it, then you may get that incongruous means "not together or inharmonious." If you didn't know the root, you wouldn't even have a guess.