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.
- For Teachers