Derivation vs. Inglection

Linguists have traditionally made a clear distinction between derivation and inflection in morphology, and What Is Morphology? takes the same approach. In reality, however, the line between the two can be blurry at times.

First, describe in your own words how to distinguish between derivation and inflection in morphology and provide examples. Then, think of at least two instances where an inflectional suffix does, in fact, change the part of speech or meaning of a word. (For example: ash—ashes. In this case, the word “ashes” can refer to a person’s cremated remains, which has a distinct meaning from the common use of “ash.”) Do you think this causes a significant problem for the distinction between inflection and derivation?