Here's why:
* Inflectional suffixes modify a word's grammatical function or meaning within a sentence. They indicate things like tense, number, case, or degree. For example, adding "-ing" to a verb creates a present participle (e.g., "walk" becomes "walking"), but it remains a verb.
* Derivational suffixes, on the other hand, can change the grammatical class of a word. They create new words with different meanings and often different grammatical functions. For instance, adding "-er" to the verb "teach" creates the noun "teacher".
Examples:
* Inflectional:
* "walk" (verb) + "-ing" = "walking" (verb)
* "cat" (noun) + "-s" = "cats" (noun)
* "happy" (adjective) + "-er" = "happier" (adjective)
* Derivational:
* "teach" (verb) + "-er" = "teacher" (noun)
* "happy" (adjective) + "-ness" = "happiness" (noun)
* "care" (noun) + "-ful" = "careful" (adjective)
In summary: Inflectional suffixes serve to adjust the grammatical function of a word within a sentence but do not alter its grammatical class. Derivational suffixes, however, can create entirely new words with different grammatical classifications.