The VERB walk inflects according to
case > he walks
tense > he walked
aspect >he is walking
The NOUN walk is inflected to denote the
plural > He enjoyed his Sunday walks
genetive > Click on the walk’s name (note the apostrophe)
Inflection is the process of adding inflectional morphemes that modify a verb's tense or a noun's number, rarely affecting the word's meaning or class. Examples of applying inflectional morphemes to words are adding -s to the root dog to form dogs and adding -ed to wait to form waited.
Sometimes a derivational morpheme is added to modify the meaning
agent noun > a walker (either a person who walks, or a walking frame)
In contrast, derivation is the process of adding derivational morphemes, which create a new word from existing words and change the semantic meaning or the part of speech of the affected word, such as by changing a noun to a verb
Sometimes the suffix -er is an inflectional morpheme, the -er is attached to monosyllabic ADJECTIVES to form its comparative. For example, hotter tells us that something has a higher temperature, and we use the suffix -est to form the superlative (hottest), the adjective hot remains an adjective even when the suffixes -er and -est are used.
The possessive PRONOUN its is always spelled its. When an apostrophe is added, i.e. it's, it changes meaning to represent the contracted form of it is or it has.
The plural form of it is not its. Similarly, we don't normally say Is, yous, hes, or shes, instead their plural equivalents are: we, you and they. The possessive adjectives: your, her and their are the only ones which accept an -s suffix, while the POSSESSIVE ADJECTIVE / PRONOUN his and its remain unchanged, e.g.
A: Whose is this cover?
B: “It's yours/hers/his (no change)/theirs”
B: “It's its” (grammatical but rather awkward to say the least.)
"Only English nouns, verbs, adjectives, and adverbs–all open classes of words–take inflectional affixes. Closed classes of words . . . take no inflectional affixes in English. Inflectional affixes always follow derivational ones if both occur in a word, which makes sense if we think of inflections as affixes on fully formed words. For example, the words antidisestablishmentarianism and uncompartmentalize each contain a number of derivational affixes, and any inflectional affixes must occur at the end: antidisestablishmentarianisms and uncompartmentalized.
(Kristin Denham and Anne Lobeck, Linguistics for Everyone. Wadsworth, 2010)
See also Wikipedia's article on Inflection