The prefixes above all mean roughly the same thing: non or not. For example ‘impossible’. The root word here is ‘possible’ meaning something which may or can be, exist, happen, be done, be used, etc. By adding the prefix ‘im’ we now have a word meaning something which may not or cannot be, exist, happen, be done, be used, etc. But why do we use ‘im’ with ‘possible’? Why is it not ‘unpossible’, ‘inpossible’, or any of the other prefixes?
Because there are rules. Now most rules in English are fairly straight forward, ‘i’ before ‘e’ except after ‘c’ types of things. This is not one of those rules. So here (after much reading and possibly some brain melting) are the rules governing which of these five prefixes to use as I understand it.
Starting with the easy stuff:
- Words which start with ‘m’ or ‘p’ take the prefix ‘im-‘ (impatient, immaterial)
- Words which start with ‘r’ take ‘ir-‘ (irresponsible, irresistible)
- Words which start with ‘l’ take ‘il-‘ (illegitimate, illegal)
Okay, kids. This is Where it Gets Complicated:
- Words of a Latin (Latinate) origin take the prefix ‘in-’ (incredible, indeterminate, inconceivable)
- Words of a Germanic (and most other languages) origin take the prefix ‘un-’ (unbelievable, unfinished, unending)
- But wait!
There are words which sometimes use more than one of these prefixes such as: uncontrollable and incontrollable. My spellcheck and the dictionary say both are correct, which do I use?With the root word being ‘control’ and deriving from the Anglo-Norman French ‘contreroller’* [verb] or The French ‘contrôle’ [noun], strictly speaking, ‘uncontrollable’ would be correct, however; ‘incontrollable’ is a recognized alternative. This happens when new words are created. The creator of the word is free to use which ever prefix they choose. In the course of adding a prefix to ‘controllable’ both were added at differing times (Uncontrollable somewhere between 1570 and 1580. Incontrollable somewhere between 1590 and 1600). In cases such as this both words are accepted into common usage and become established in the language. You may use either of them, though be aware ‘uncontrollable’ is the preferred form for most native English speakers.*Some would argue the Medieval Latin ‘contrarotulare’ as the root for ‘contreroller’ thus ultimately giving a Latinate origin to ‘control’. Personally I am unclear in this regard (more research required). It bares noting as a reasoning behind the use of the ‘in-’ prefix in this instance.
- There’s more!
The word ‘sure’ is derived from Middle English ‘sur’ which came from Middle French ‘sur’ which traces back through Old French ‘seur’ which has it’s origin in the Latin ‘securus’, so why do we say ‘unsure’?This is either a similar case to ‘uncontrollable’ vs ‘incontrollable’ or stems from ‘sure’ also being related to ‘sicher’ which traces from the Old High German ‘sichur’. It is my belief it is the Germanic origin at work here given that we do say ‘unsure’ but we also say ‘insecure’ which has the same root from Latin ‘securus’
Confused yet? So am I.
So, what do we, as writers struggling to find words and make sure we are using the words we find correctly and really not wanting to spend hours researching the origins of every single word we use, do? In most cases you can look them up. A quick google of “define:<your word here>” will generally let you know if you are using the correct prefix. If your first try doesn’t produce any results (for example if you searched for ‘inclear’, while the correct form is ‘unclear’) try searching again with a different prefix. If you have run through all five of these prefixes and still get no results, congratulations! You have created a new word! But, remember, you are helping to shape a language. Say the word out loud a few times, make sure it flows, try it out in a sentence or two and whenever possible try to stick to the rules above.