The Suffixes -able and -ible
First, since we are dealing with suffixes, they come late enough in the word that if you can spell the rest of the word, you can find the correct form in the dictionary. So they are easy to look up.
However, second, if you are stranded without a dictionary, -able is about six times more common than -ible, so if you have to guess, guess -able.
Third, as the next four lessons will show, there are some patterns that can be quite helpful.
2. In the following table fill in the unshaded blanks. Then answer the question at the end of the table:
3. Do verbs that form nouns with -ation form adjectives with -ible or with -able? ___________.
That leads to our first useful generalization: Stems that form nouns with <ation> take -able to form adjectives
Teaching Notes. The complications that we are trying to sort out here arise from a number of complications that occurred hundreds of years ago when words with -able and -ible were brought into the English language, usually from French and Latin. In general, the forms with -ible came directly from Latin, while those with -able came by way of French. But -able became the preferred form in English so that some words originally with -ible were respelled with -able, and -able was used with new adjectives based on native verbs, like unspeakable.