Ible, able
In English, -ible and -able are (derivational) suffix morphemes that indicate capability.
They can be phonetically distinct, but most people produce and hear it fairly unconsciously in spoken language, so the spelling and pronunciation is not something that we often think about.
when to use -ible or -able
Most people are pretty accepting of mixing these in writing, or (seemingly) know it for common words through seeing it a lot.
The technical rule involves the origin of the word:
If the origin is Latin, and the Latin verb form ends in ere or ire, you get the ible form.
But since most of us don't speak Latin, here are some decent rules of thumb:
- you often use -ible when the base is not a complete/standalone word, for example:
- cred+ible
- poss+ible
- you often use -able when the base is a complete word (which seems the more common case), for example:
- laugh+able
- comfort+able
- rock+able
There are various irregularities and exceptions to those, including:
- inevitable
- flexible
- responsible
- irritable
- permeable
eable versus able
That is, when the root ends in an e, should you remove it or not?
Usually, yes. Consider:
- advise-e+able = advisable
- debate-e+able = debatable
- adore-e+able = adorable
- wade-e+able = wadable
- unshave-e+able = unshavable
There is a fuzzy exception: If removing the e would result in a significantly different pronunciation (or be different words and/or rather confusing), leave it in. Consider:
- peaceable, not peacable
- twineable, not twinable
It seems there are a good number of things used in both forms, such as:
- sizable, sizeable
- tradable, tradeable
Presumably(verify) these exist because people play safe and don't always omit the e.