Það er búið að hita teið.

Here það is a dummy subject (an “it/there” that doesn’t refer to anything concrete), similar to English it is done or it has been done. Icelandic often uses það to build impersonal sentences where no real subject is named:

• Það er búið að ... = it is finished to ... So það doesn’t mean a specific “that” here; it’s mainly structural.

Búið is the past participle of búa (in this construction), used in the idiom að vera búinn/búin/búið að + infinitive, meaning to have finished doing / to be done (with) doing. So the structure is:

• er (present of vera, “is”)
• búið (past participle, “finished/done”)
• að hita (infinitive phrase, “to heat”) Together: It is finished to heat the tea → idiomatically: The tea has been heated / Someone has heated the tea (already).

Because the participle agrees with the grammatical subject. Here the subject is dummy það, which is neuter singular, so you get búið. If you use a real person as the subject, it changes:

• Ég er búinn að hita teið. (male speaker)
• Ég er búin að hita teið. (female speaker)
• Ég er búið að ... is generally only for neuter things/children, and is uncommon for adults.

It’s not a straightforward passive form like Teið var hitað (The tea was heated). It’s an impersonal completion construction:

• Það er búið að + infinitive It often functions like a passive in meaning (the doer is not mentioned), but grammatically it’s built with vera
  • participle búið
    • infinitive clause.

Common alternatives:

• More passive-sounding: Teið hefur verið hitað. (The tea has been heated.)
• Simple active: Ég hef hitað teið. (I have heated the tea.)
• Simple past active: Ég hitaði teið. (I heated the tea.)

Formally, er is present tense, so the sentence is “present” in form, but it expresses a completed action (similar to an English perfect/result meaning). If you move it to the past:

• Það var búið að hita teið. = It was already done (the tea had been heated).

In this construction, búið is followed by an infinitive phrase introduced by að:

• búið að gera = done/finished doing So að hita is “to heat”. You typically keep að here.

teið is te (tea) with the definite article attached: the tea.

• te = tea (indefinite)
• teið = the tea (definite)

It’s neuter singular. The definite ending for many neuter nouns can be -ið.

Negation:

• Það er ekki búið að hita teið. = The tea hasn’t been heated (yet).

Yes/no question (inversion):

• Er búið að hita teið? = Has the tea been heated / Is it done heating the tea?

Literally it means heat the tea (raise its temperature). It does not necessarily mean brewing/making tea from scratch. If you mean “make tea” (prepare it), you’d often use something like:

• gera te = make tea
• brugga te can be used in some contexts (brew), but gera te is very common.