There are many suffixes to learn.  Two suffixes which are easily confusing are –able and –ible because they sound alike. 

Words such as agreeable, comfortable, horrible, visible, for example, usually cause problems.  The –able words (adjectives) have related –ably forms (adverbs) and the –ible words (adjectives have related –ibly forms, as in agreeably, comfortably, horribly, visibly, etc.

The following two points about –able and –ible simplify the decision as to which spelling is correct:

To figure out that demonstrable should end in –able, think of the well-known noun demonstration, considerable from consideration.  Similarly, you can figure out admissible from admission, destructible from destruction, etc.

When a negative prefix, such as un-, non-, dis-, im-, in-, or ir-, is added to the adjective or adverb, the basic fact about –able or –ible and –ably or –ibly remains unaffected.  Remember that Point 2 is not a rule.  There are certain exceptions to the “a-for-an-a and i-for-an-i” guideline.  It is a helpful guide to the spelling of a great many –able and  –ible words.  For example, correctable and detectable are the preferred spellings of the adjectives despite the noun forms correction and detection.  Sensible is correct, in spite of sensation; and predictable is correct, in spite of prediction.   These are only four major exceptions; there are several others.