–able is added to Whole Words, –ible to Stems

Another helpful guide to spelling these adjectives and adverbs is that the suffix –able is often added to whole words such as agree (agreeable); to whole words minus their final –e, such as cure (curable); and to whole words whose final –y is changed to –i, such as rely (reliable).  The suffix –ible, on the other hand, is often added to apparently meaningless groups of letters and to stems or roots what are not whole words, as in horrible, terrible.

Exception: