Generally speaking, we don’t consider inflectional forms of the same stem to be different words, but to be different forms of the same word. The collection of all of the inflectional forms of a root are called the paradigm for that word. For example, the row (1a) is the paradigm for the noun cat, row (2a) is the paradigm for the verb walk, and row (3a) is the paradigm for the adjective red.

We can also talk about an inflected word form as having morphosyntactic features. For example, cat would have the feature [+singular] and cats would have the feature [+plural]. Morphosyntactic features classify words into groups based on similar behaviour. For example, English words with the feature [+plural] share the behaviour of being marked with a plural marker and having the meaning of ‘more than one.’

In contrast, derivational morphemes create words of new parts of speech or words with new meanings. For example, the verb scare can be changed into a new word, the adjective scary, by adding the derivational morpheme -y. Although scare and scary share the same root scare, they are different words. The verb scare refers to an event and belongs to a verbal paradigm and can take verbal inflectional morphology, such as scared and scaring, while the adjective scary refers to a quality and belongs to an adjectival paradigm and can take adjectival morphology, such as scarier and scariest.

Derivational morphemes don’t always change the part of speech of the word, though. Morphemes that change the meaning of the word enough that we want to call it a new word, even if it keeps the same part of speech, are still called derivational morphemes. For example, the prefix re-, attaches to verbs to form new verbs that mean to perform the action again, as in (4). Both scared and rescared are verbs.

Sometimes derivational morphemes do both; they change the meaning substantially and change the part of speech. For example, adding the suffix -er to a verb creates a noun that identifies the person who performed the action, known as an agentive noun, or adding -able to a verb creates an adjective meaning that the action is possible to perform.

Derivational word forms based on the same root belong to the same word family, but each has their own, separate, inflectional paradigm. For example, the word family of scare includes the verb scare (5a), the adjective scary (5b), the noun scare (5c), and the adverb scarily (5d). It also includes more complex forms such as the repetitive verb rescare (5e), the agentive noun scarer (5f), and the adjective scareable which expresses possibility (5g). The word family even includes words containing multiple derivational morphemes, such as the word scariness in (5h), which consists of three morphemes: the verb root scare, the adjective-forming suffix -y (spelled with an <i> when it is not at the end of the word), and the noun-forming suffix -ness.