What is inflectional morphology and how does it differ from derivational morphology?

The core of this answer is really about how the semantic meaning of the word changes after it has been altered. Inflectional morphemes don't undergo the same kind of drastic change that derivational morphemes do. By this, I mean that a word like 'cat' can be inflected for plural form by simply adding the suffix -s to the end (i.e. 'cats'). When you do this, you are still dealing with 'cat' as a noun and simply adding more of it to the count. Likewise, the infinitive 'to dance' can be inflected for person (he DANCES) and tense (he DANCED).

However, with a derivational morpheme, you are altering the class or category of the word entirely. For instance, if I gave you the infinitive 'to inform', we could transform it into a noun by adding the suffix -ation, which would give us 'information' (after you deleted the 'to' infinitizer). If you took the noun 'love' and transformed it into an adjective, it would be 'lovable' with the addition of the suffix -able.

Hopefully this clears up some of those differences for you!