The suffix “-able”
Suffix is an affix mostly paced in the end or after a word’s original root. It takes on many cases to describe a word either in verb or adjective. For example running notice that –ning is the suffix in this word while beautifully, take the root word of beautiful and the suffix –ly as we break down the words.
The rule that applies most closely to "avail" is that it is almost always used in conjunction with another word to enhance the second word's initial meaning.
Hope this helps! :)
