There are some things that most of us readers and speakers of English just know. The letter q is always followed by u. I don’t care what the Scrabble Dictionary says. Though si sometimes sounds like sh (session), I don’t remember ever seeing anyone trying to use si for the sh sound at the beginning of a word (siip or siop rather than ship or shop).
Then there are things good spellers know, but may not be able to explain. The other day someone asked me, “How do you know if a word ends in -ible or -able?” My very helpful reply was, “I just know. If I’ve seen it, I’ll remember.” I really had never thought about it. Just a few days later I happened to read the explanation and it made perfect sense. The rule doesn’t hold true in every instance (you knew I was going to say that, right?), but here’s the “usually.” A word that ends in -ible is usually of Latin origin. If you take off the -ible you will have part of the Latin root, but not a complete English word. For example, terrible. If you take off the -ible you are left with terr. That doesn’t mean anything in English, though it is the first part of terrēre, which means to frighten in Latin. The same holds true for horrible. Also for illegible, incredible, ostensible, plausible, and visible.
Though this doesn’t work for accessible, it does work for inaccessible. In the same way, destruct in the word destructible is a complete word, but indestruct, as in indestructible, is not. That might be a good way to remember some of these exceptions. Add a “not” prefix, in- or un-, to a questionable word and see if that word without the -ible or -able is still a complete word. This would help you out with flexible, controvertible, and resistible.
The test, then, for the -able words would be the same. If the word is still a complete English word without its suffix it is likely that you would use -able. Attainable, avoidable, breakable, comfortable, considerable, dependable, disagreeable, foreseeable, laudable, laughable.
Let’s look at some of the exceptions for the -able words. There are a couple of not-so-obvious reasons for many of them. The first is for the words that have had their tails lopped off in order to form a more pronounceable word. I can’t say I know when or how this happened (not because it’s a secret, but because I don’t know), but in most cases, I think we can be thankful that it did. What if we said equalable instead of equable (obviously equa is not a complete word)? How about appreciatable instead of appreciable; calculatable instead of calculable; impenetratable instead of impenetrable; or negotiatable instead of negotiable? I suppose we would be used to it, but somewhere along the line someone thought better of the longer forms. I haven’t investigated this thoroughly, but perhaps these tailless words are another good clue for choosing between -ible and -able.
Another rule that makes many of these -able words tricky is that they seem to be incomplete words because the e at the end is missing. The rule here is that we take off the final silent e before adding an ending that starts with a vowel. This works for words like advisable, deplorable, disputable, and imaginable. We do this because if we leave the e on the end of those words, we end up with the inscrutable diphthong ea before the suffix, which would necessitate figuring out if the word should be pronounced imaginable or some variation on the other possible sounds of ea: imagineeble or imaginehble.
The exceptions to taking off the final silent e may seem dizzying at first, but they make sense in light of some of the other rules we have already gone over. You do not take off the e if doing so breaks another spelling rule. Because a silent e often changes the sound of g from a hard gh to a soft j, changeable, knowledgeable, and manageable keep the e to signal that those words keep the j sound for the g.
Likewise for c. An e often changes the hard k sound to the soft s, so noticeable, peaceable, and serviceable keep the e, lest we wonder if these words are to be pronounced notickable, peackable, and servickable.
Another rule tells us to change a y at the end of a word to an i before adding an ending that starts with a vowel (unless you are adding -ing). So, variable, pitiable, and reliable don’t look like complete words when you take off the -able, but they would be if you were to put the y back.
The e on the end of argue and value keeps those words from ending in u, which rule we’ve discussed before. The e at the end of believe, conceive, live, and love keeps those words from ending with a v, also a no-no in English. Since that is no longer a problem once the -able is added, the e goes away in arguable, valuable, believable, conceivable, livable, and lovable.
We’re making tangible, laudable progress. Next time, I hope to be available to answer more formidable questions about some seemingly inexplicable, indiscernible spelling rules.