It is a third-person, singular neuter pronoun (nominative (subjective) case and oblique (objective) case) in Modern English.
Usage
The word and term 'it' can be used for either a subject or an object in a sentence and can describe any physical or psychological subject and/or object. The genitive form its has been used to refer to human babies and animals, although with the passage of time this usage has come to be considered too impersonal in the case of babies, as it may be thought to demean a conscious being to the status of a mere object. This use of "it" is also criticized when used as a rhetorical device to dehumanize their enemies, implying that they were little more than non-human animals. The word remains in common use however, and its use increases with the degree to which the speaker views an object of speech as impersonal. For example, someone else's dog is often referred to as "it", especially if the dog isn't known by the speaker, or if the dog's gender is unknown. A person would rarely say "it" when referring to his/her own cat or dog. Examples:
"It" is still used for idiomatic phrases such as Is it a boy or a girl? Once the gender of the child has been established, the speaker or writer then switches to gender-specific pronouns.
Some people propose using "it" in a wider sense in all the situations where a gender-neutral pronoun might be desired. The advantage of using an existing word is that the language does not have to change as much. The disadvantage is the possibility of causing offense. This usage of it is currently very rare, and most commentators feel that it is unlikely to catch on. Samuel Taylor Coleridge was one early advocate of this.
One author who consistently wrote in this manner was the children's author E. Nesbit, who often wrote of mixed groups of children, and would write, e.g., "Everyone got its legs kicked or its feet trodden on in the scramble to get out of the carriage." (Five Children and It, p. 1). This usage (in all capital letters, as if an acronym) also occurs in District of Columbia police reports.
In earlier Middle English, arising from Old English, the pronoun was hit (similar to Dutch "het" and West Frisian "hit" with the same meaning), with the unaspirated it being an unaccented form. The genitive was his, with the new form its only arising by analogy in later Middle English.
The pronoun it also serves as a place-holder subject (dummy pronoun) in sentences with no identifiable actor, such as "It rained last night.", "It boils down to what you're interested in.", or the impersonal "It was a dark and stormy night."