Inflectional morphology, refers to the way in which bound grammatical morphemes combine with stem to be realize as grammatical words.
While, derivational morphology describes the way in which bound lexical morphemes combine with stem to be realized as lexical words.
Inflectional morphology doesn’t change the semantic meaning or category (part of speech) of the stem. For example; a noun remains noun and a gender remains gender, hence, it will only change the pronunciation.
Whereas, derivational morphology after combining with the root word it changes the semantic meaning. For Example; modern – modernize, read- readable, private- privacy etc.
Inflectional morphology of English describes the bound grammatical morphemes of the same language and the grammatical rules in which they figure, and the paradigm they form. Also, the various phonological and orthographic forms into which they are realized.
In contrast, derivational morphology of English describes the bound lexical morphemes of English, the categories of the stem with in which they combined, the categories of the resulting forms. Therefore, in this process the meaning changes and the phonological, orthographic shapes these morphemes they acquire.
Therefore, the major difference between these two morphologies is that the inflectional morphology deals with the creation of new forms of same word. Whereas, the derivational morphology deals with the creation of new words only.