Emily Dickinson Lexicon

it (-s, -'s), pron. [OE hit; third person singular neuter pronoun.] (webplay: affair, arms, ascertained, asked, betrayed, charmer, Christ, clause, diligence, earth, figure, general, heart, indefinitely, Judas, life, matter, nails, name, person, question, rains, saint, sentence, sinner, snows, state, substitute, teeth).

1. [Anaphoric reference, pointing back to a previously-mentioned noun in the text] that thing; the one mentioned.
2. [Cataphoric reference, pointing forward to a subsequent noun in the text] the event; this thing.
3. [Exophoric reference, pointing to a noun not mentioned directly in the text; “It sifts” = “Snow sifts”] this; the thing being described.
4. [Phatic “dummy” subject; “It will be Summer” = “Summer will happen”] someday; at some time.