Which word part can be added to a base word to change the number or tense?

Derivational endings, while also added to base words, change the meaning or part of speech of the base word. For example, adding "-ly" converts an adjective into an adverb (e.g., "quick" to "quickly"). Unlike inflectional endings, derivational endings do not merely indicate grammatical variations like tense or number but create entirely new words.

Negative prefixes, such as "un-" or "dis-," are added to the beginning of base words to create a word with an opposite meaning. For instance, "happy" becomes "unhappy." These prefixes affect the meaning of the base word but do not alter tense or number, hence not serving the purpose of changing grammatical forms.

A root word is the base form of a word without any affixes. It serves as the foundation for building other words but does not itself indicate changes in tense or number. For example, "act" is a root word, but it requires an inflectional ending, like "-ed," to convey past tense.