Breakdown of Það er búið að endurstilla lykilorðið.
vera
to be
það
it
lykilorðið
the password
endurstilla
to reset
Questions & Answers about Það er búið að endurstilla lykilorðið.
What does the construction Það er búið að + infinitive mean?
It’s an impersonal, resultative construction meaning that something has already been done. Here, Það er búið að endurstilla lykilorðið corresponds to English like The password has been reset or They’ve reset the password (agent not specified).
Why is it er and not hefur?
Because búið here is the neuter form of the adjective búinn (done/finished) used with vera (er), not a perfect tense with hafa (hefur). If you want a standard passive perfect, you’d use: Lykilorðið hefur verið endurstillt.
Who is the subject here? What does Það do?
Það is a dummy/expletive subject (like English “it/there” in certain sentences). The construction is impersonal: it doesn’t name who did the action, just states that it’s been done.
Can I mention who did the resetting?
Generally not with the er búið að construction. If you need an agent, switch to the passive or active: