The English language has a rich storehouse of words accumulated through different innovative methods. Adding prefixes to words, already in use, is one such method.

A prefix is a type of affix attached to a stem which modifies the meaning of that stem. The prefix un when added to happy makes it unhappy just as the prefix pre when added to historic transforms it into prehistoric. The word prefix is itself made up of the stem fix, meaning attach in this case, and the prefix pre, meaning before, both of which are derived from Latin roots.

A sound knowledge of the sources and meaning of the very commonly used prefixes considerably enhances the word power of a learner of the English language.

An interesting point worth mentioning is that most prefixes have crossed over from Latin and Greek to become parts of the English language. For example the prefix ab in abduct, ad in adjoin and bio in biology owe their origins to these languages.

It is also important for you to know that initially prefixes were not integral parts of the words to which they were affixed. They were usually joined to stem word by means of a hyphen. For example, strange as it may seem now, today was once written as to-day and unusual as un-usual. Likewise, subcontinent was until recently written as sub-continent.

The modern trend is to do away with unnecessary hyphens. However, hyphens are retained in the following instances

1. The stem word is a proper noun. For example all-American or pro-Pakistani
2. All words starting with the word self retain the hyphen such as self-interest, self-respect and self-employment. The exceptions are selfless and selfsame.
3. Hyphens are also retained with re when it means again and where joining the prefix to the stem may cause some confusion. For example, “I need to re-press this shirt”. This sentence would take on a different meaning if the word re-press is written without a hyphen.
4. A hyphen between the prefix and the stem is not to be removed where the prefix ends in and the stem begins with the letters a or i. For example semi-invalid or ultra-ambitious.
5. A hyphen is considered a necessary fixture between the prefix ex and its stem. For example ex-wife or ex-student.

Most standard language textbooks give you quite a thoroughly worked out list of these prefixes. Each prefix has its own meaning which in turn affects the meaning of the word, to which it is affixed, quite radically in most cases.
The prefixes un, in, ir, im, il, dis and anti are all meant to change the meaning of a word to its total opposite. For example, rational becomes irrational; patient becomes impatient; satisfied becomes dissatisfied; kind becomes unkind; logical becomes illogical and climax changes to anticlimax.

Then there are some prefixes that are used to quantify the elements in a word. For example, ambi meaning both in ambidextrous defines a person who can use both hands for writing and other activities with equal ease. Some other prefixes that serve a like purpose include bi meaning two in binoculars, demi meaning half in demigod, hemi meaning half in hemisphere, mono meaning one as in monologue, octa meaning eight in octopus, multi meaning many as in multimillionaire, semi meaning half in semicircle and tri meaning three in tripod.

The main purpose of giving these examples is to evoke further interest in the use of prefixes. One needs to be fully acquainted with the meaning and sources of the prefixes used in the English language. This comprehension will certainly help to enhance one's written and spoken skills.