-able or -ible
The two endings sound identical, so the ear cannot choose. What comes before the ending usually can.
The rule
-able and -ible sound identical, so the ending has to be learned rather than heard. -able is the commoner of the two, and usually follows a part that can stand as a word on its own.
Take the ending off and look at what is left. If it stands as an English word on its own, the ending is usually -able: read, readable; comfort, comfortable; depend, dependable. If it does not, the ending is often -ible: visible, audible, plausible, terrible.
A word that drops a final E before the ending still counts as a whole word: love, lovable; believe, believable.
A soft C or G before the ending tends to take -ible (legible, eligible, forcible), and a hard one -able (navigable, applicable).
Where it stops holding
The test leans; it does not decide. Some whole words take -ible (flexible, sensible, collapsible, digestible), and some parts that are not words take -able (probable, inevitable, formidable).
Background
Choosing an ending by the part it attaches to is reasoning from a word's structure, which the research finds teachable. That research is on how people learn to spell.
Try it on real words
Five words, read aloud and typed from memory with autocorrect off. About a minute, no signup.
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