When to Add -ible and When to Add -able

Here's a tricky question for you:

Why do some words add the suffix -ible and others -able?

Why indeed.

A good rule of thumb is that if the root word is a complete word, you add -able
e.g. accept - acceptable; laugh - laughable; suit - suitable and so on.

If the word ends in y, change it to i e.g. justify - justifiable, and if it ends in e, you usually (but not always) drop the e.
e.g. believe - believable.

But ... if the consonant preceding that vowel is a g or a c, you keep the e. If you don't, the consonant would become hard, and the word would sound odd.
e.g. notice - noticeable; knowledge - knowledgeable.

And, if removing the e would change the pronunciation of the preceding vowel then you leave the e.
e.g. like (long i) - likeable; sale (long a) - saleable.

If the root is an incomplete word, you add -ible
e.g. vis- visible; tang- tangible; cred - credible.

Remember this by the two i's: Incomplete -ible.

That's a pretty easy way to work it out, don't you think? You'll find more helpful writing tips at www.write101.com