Prefix is a type of affix that is situated in the beginning of the word, before the stem.

Examples of words with prefixes in sentences:
She is unhappy because summer is over. 
Angel speaks Filipino, Chinese, Japanese, Spanish and Enlish - she is a polyglot. 
Sometimes school is overwhelming. 
Monotheism is when people believe in one God.
There is a malfunction in my computer.
A painter prefer to draw irregular shapes.
Her disappearance made everybody worried.
Students should practice by retaking the most difficult exercises.

There are many more of prefixes: a-, auto-, bi-, co-, contra-, extra-, il-, im-, pre-, post- etc. 
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