This sentence is in the passive voice. To form the passive in Danish, you have two options:

The -s passive

To form the passive, you can add an -s to the verb.

• Manden åbner døren = Døren åbnes (af manden)

• Kaptajnen sejler skibet = Skibet sejles af kaptajnen

• Katten spiser musen = Musen spises af katten

This also works in the past tense, although it is rarely used:

• Manden åbnede døren = Døren åbnedes (af manden)

• Kaptajnen sejlede skibet = Skibet sejledes af kaptajnen

• Katten spiste musen = Musen spistes af katten

This sentence could have been: Dørene åbnedes i går, although this construction is rare.

The blive passive

This is the most common way to form the passive. You use an inflicted form of blive + the past participle.

• Manden åbner døren = Døren bliver åbnet (af manden)

• Kaptajnen sejler skibet = Skibet bliver sejlet af kaptajnen

• Katten spiser musen = Musen bliver spist af katten

As you can see above, blive is used to form the passive (referring to the action of opening the door). If you instead use være + past participle, you are referring to the state of the door as having been opened.

• Dørene blev åbnet i går = Someone opened the doors yesterday

• Dørene var åbnet i går = Yesterday, the doors were in a state of having been opened by somebody

• Dørene var åbne i går = Yesterday, the doors were open (here, the adjective åben is used rather than the past participle of the verb.)