-it-

-it- is the passive participle of the past: the root's action has been done to someone or something, and it is finished. fermi gives fermita (closed), skribi gives skribita (written), perdi gives perdita (lost). With esti it makes the completed passive: La libro estis skribita antaŭ cent jaroj. Most of the English past participles that end in -ed or -en land here.

The nuance worth holding on to is that -it- is about the result. La pordo estas fermita says the door is shut now, because someone shut it; estas fermata would say the shutting is going on. Put ne in front and you have the everyday "un-" adjective: nesolvita, unsolved.

It mirrors -int- on the active side: fininta is the one who finished, finita the thing finished. With -at- and -ot- it completes the passive set, set out in the passive article.