The word it is a pronoun. When I add an s to it, does it change the word class?
For example in the following sentence:
The gift is still in its box.
My questions are:
Does the "S" change the word class?
Is the "S" an inflectional or derivational morpheme?
Its is in your first example the possessive determiner of the personal pronoun it. See this page by Cambridge Dictionary, which states the following about it:
personal pronoun: it
possessive determiner: its
possessive pronoun: its (this one is rarely used)
The added s is not an inflectional affix (unless one would treat it as a noun), because there are only 8 in the English language and those 8 do not correspond with this one. If it is seen as a noun, it could be argued that the added s is a noun possessive, which is one of the 8 inflectional affixes listed on that page.
According to this Wikipedia page it (and other pronouns) is (are) a functional morpheme(s).
This is just a supplement to the excellent answer by sumelic.
For completeness:
No, adding an 's' to 'it' to produce 'its' does not result in a change in class. Both it and its are pronouns.
This 's' is an inflectional morpheme.
Discussion
Both CGEL and ComGEL would say that its is just the pronoun it in the genitive case. And as far as what 'case' is, according to CGEL (p. 455),
The term case applies in the first instance to a system of inflectional forms of a noun that serve to mark the function of an NP relative to the construction containing it.
(CGEL treats pronouns as a subcategory of nouns, whereas ComGEL does not; if you don't want to take pronouns as a subcategory of nouns, just replace of a noun in the above by of a noun or a pronoun.)
These two grammars do not use the term 'inflectional morpheme', but from the above, it follows that 's' in its is precisely that.
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
The word it is a pronoun1. The word its is also a pronoun2. So the word class is the same.
The s is an inflectional morpheme, like the 's used in the spelling of the possessive form Sally's of the noun Sally. In fact, although the spelling is different in standard English, it could be argued that the s in its is the same inflectional morpheme as 's. In more abstract analyses, the same morpheme might even be considered to be present in words like her and my despite the completely different phonological realization: the idea that one morpheme may have completely different phonological realizations in different words is an extreme example of the concept of "allomorphy". The extended version of this concept is often illustrated with the example of English plurals: some people argue that even though there are several distinct ways of marking the plural forms of nouns in English from a phonological perspective, all plural nouns can be considered to contain the same plural morpheme (variably realized as things like /z/, /s/ /ɪz/, ∅, /ən/, ...). You can see a more extensive description of this in the following presentation: http://ling.uni-konstanz.de/pages/home/plank/for_download/SHE-I-WS11-12/2-Allomorphy.pdf
Even if you don't use this conception of allomorphy, it's still clear that its is an inflected, not a derived form of it. You can see pronoun forms described as inflections in "Cases, Inflection, and Pronouns" by Tony Jebson.
Note that pronouns may be considered a subcategory of nouns (pronouns obviously don't behave the same as either proper nouns or common nouns, but that can be explained in terms of pronouns not sharing the properties that are specific to these other subsets of nouns, rather than pronouns not sharing the properties that are characteristic of the overarching category of nouns.)
Some people call words like my your its "possessive adjectives" or "possessive determiners" rather than "possessive pronouns", reserving the latter term for words like mine yours, but there are several arguments for considering both sets of words to be pronouns. Forms like her his its can take previously-mentioned nouns as antecedents; they can also be used as the subject of the non-finite -ing form of a verb (at least for some speakers--there are some speakers who apparently think that this construction sounds strange in many contexts).
