Words ending -able and -ible

 

Two common endings are -able and -ible. But how can you tell which one to use?
The difficulty is that we pronounce the leading vowels very similarly: we make a sort of -ugh- sound
and you can't hear whether it is -a- or-i- .
There are no simple rules to follow.
But here are three tips that you may find useful:

• If the base word is not altered in any way then add -able.
  This works most of the time:
  understand —> understandable,
  predict —> predictable,
  comfort —> comfortable, and so on.
• There are many more words ending -able than -ible.
  I counted 900 -able words and 200 -ible words.
  So, if you make a guess and choose -able
  you're probably right 4 out of 5 times!
• The best advice I can give you is to learn the most common -ible
  words and use -able for the rest.
  Very many of the -ible words are so rare that you'll never use them.
  Have you ever written fungible for instance

Here are the most common -ible/able words: 

convertible       be converted

predictable       be predicted

durable           last a long time

viable            to be made to work or put into effect

variable          vary

edible:	fit for eating
credible:	can be believed
audible:	can be heard
legible:	easy to read
negligible:	of no importance
terrible:	causing terror
horrible:	awful
feasible:	can be done
visible:	can be seen
sensible:	reasonable
responsible:	trustworthy
possible:	liable to happen
digestible:	can be digested
flexible:	easily bent
permissible:	allowable
compatible:	suitable

Many of them have negative forms by adding in-, il-, ir-, or im- . For example: inedible, illegible, irresponsible, impossible.