How can you tell whether to use the suffix "ible" or "able"? Generally, if the word can stand alone, such as "reason", add -able. If not, add -ible (as in "horrible"). As ever in English, there are exceptions, however!
This is the third and final quiz on suffixes. So far, you've had a chance to practice using suffixes to make grammatical sense in sentences as well as recognising which suffixes work with which root words. This quiz gives you the opportunity to practise spelling the suffixes -ible and -able. Because the i and the a make a schwa sound (ǝ), it's impossible to tell which spelling to use by saying the word aloud. Instead, consider the rule above. "Credible", for instance, cannot stand without its suffix. "Cred" is not a word (although it is a commonly used abbreviation for "credit"). The word "credible" is related to "credit", "creed" and "credulous" - you can see the similarities in their roots. "Dependable", on the other hand, is a genuine word without its suffix. "Depend", therefore, takes -able, rather than -ible. Bear this rule in mind and you won't go far wrong with spellings.
Take this quiz for more practice with suffixes.