A prefix is a word part attached to_ a the beginning of a root word b the middle of a root word
c the end of root word

It's to know your purpose before you read something because a purpose can:

Suppose you need to verify how correctly use commas you open ur english textbook and scan the chaper titles in which one would most likey find the information you need

Some one drawing water and my theater placed my hand under the spout. as the cool stream gushed over one hand she spelled into the other the word water , first slowly , then rapidly . i stood still ,my whole attention fixed upon the motions of her fingers . suddenly i felt misty consciousness as of soothing forgotten --a thrill of returning thought and somehow the mystery of language was revealed to me this passage from the stoyof my life is an wxample of which element of plotclimax action