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).