A trawl through the internet offers some clues as to possible rules governing which spelling to use.
Bhacharada.com lists the following points:
The site essentially says that 1) -able is preferred because it is statistically more probable and that 2) someone who is writing in a hurry should take the time to search for a related noun ending in -ation to determine whether the suffix is -able, which is already the preferred form. These rules don't seem to be terribly helpful in most cases because for words like understandable or responsible, without corresponding -ation nouns, it's a guess because the rule doesn't state that adjectives without an -ation form can't have -able or -ible as a suffix.
Point 1: –able is the basic form. Many more words end in –able than in –ible. When in doubt, and if your dictionary is temporarily unavailable, use –able (or –ably).
Point 2: An a for an a and an i for an i: If the adjective is closely related to a noun that ends in –ation, the adjective is almost certain to end in –able; if a related noun ends in –ion instead of –ation, the adjective is pretty sure to end in –ible.
Englishclub.com states that
The -ible ending is for words of Latin origin. There are about 180 words ending in -ible. No new words are being created with -ible endings. Here are the most common examples:
Okay, so both endings are for Latin-derived words. That doesn't help much.
The -able ending is for:
* some Latin words, for example: dependable
* non-Latin words, for example: affordable, renewable, washable
* new (modern) words, for example: networkable, windsurfable
The site also provides a rule of thumb:
Interesting, but there seem to be quite a few exceptions to that rule. To the ones listed, we might add "destructible," "combustible," "resistible," among others. So the rule seems rather flawed, at best.
Rule of thumb: This rule can help you decide the correct spelling. It works most (but not all!) of the time. Remember, if you are not sure about a word, it is probably best to use a dictionary. Here is the rule:
* If you remove -able from a word, you are left with a complete word.
* If you remove -ible from a word, you are not left with a complete word (note that accessible, contemptible, digestible, flexible and suggestible above are among the exceptions to this rule).
The Pennington Publishing Blog goes even further, stating
but concedes the following common exceptions:
End a word with "able" if the root before has a hard /c/ or /g/ sound (despicable, navigable), after a complete root word (teachable), or after a silent e (likeable). End a word with "ible" if the root has a soft /c/ or /g/ sound (reducible, legible), after an "ss" (admissible), or after an incomplete root word (audible).
That rule seems awfully involved and confusing, especially when we consider that many words fall into two categories. Take "pass," which is a complete root word, so it should end in -able. However, it also ends in the letters -ss, suggesting that the adjectival form should be "passible." In this case, the first guideline applies, and the word is indeed correctly spelled "passable," yet there is no way a native or non-native speaker of English would know this based on the stated rule.
Exceptions to the rule: collapsible, contemptible, flexible, formidable, indomitable, inevitable, irresistible, memorable, portable, probable
Ultimately, it seems that the best way to deal with words ending in -able and -ible is to buck up and memorize them. And, as with the case of adjectives ending in -ic and -ical, words ending in -able and -ible will probably continue to force people to head to their nearest dictionary to dispel the almost inevitable (one of those pesky exceptions!) doubts.
(Posted by language fan and friend Silas McCracken.)