What is the function of the prefix in "impossible?"
A prefix is a word part in front of a base or root word. The purpose of the prefix, “im--”, is to negate the meaning of the root or base word, “possible.” The true definition of im—is “not”. Other prefixes that mean “not” are un--, in--, il--, ir--, and dis--. These prefixes give adjectives, nouns, and verbs a negative meaning.
Memorizing the meaning of prefixes and suffixes is really handy when you are confronted by a word you don’t know. Breaking the word into its parts will help you discover the meaning when you know what the prefix or suffix means. The memorization of the meanings of prefixes and suffixes will help you build your vocabulary and increase your reading comprehension.
In this word, the prefix is the "im." In this case, the prefix "im" is affixed to the word "possible." The prefix negates the meaning of the word because "im" is a prefix that means "not."
If you think of other words with the "im" prefix, you will see that it always means this. Something that is "impenetrable" cannot be penetrated. Something that is "implausible" is not plausible (which means likely). If something is "improbable" it is also not likely or probable. Similarly, if something is "impossible" it is not possible. That means that it cannot happen.