Here's why:
* Free morpheme: A free morpheme can stand alone as a word with meaning.
* Bound morpheme: A bound morpheme cannot stand alone and needs to be attached to another morpheme to have meaning.
In "disgraceful", "dis-" is a bound morpheme (prefix) that means "not" or "the opposite of". "Ful" is also a bound morpheme (suffix) that means "full of".
Therefore, the only free morpheme in "disgraceful" is grace.