Feen können nur von Kindern gesehen werden.
Why is werden used here instead of sein as in
Feen können nur von Kindern gesehen sein.
German Language Stack Exchange is a bilingual question and answer site for speakers of all levels who want to share and increase their knowledge of the German language. It's 100% free, no registration required.
Sign up to join this communityGerman has two passive voices. The standard one –Vorgangspassiv– is built with the werden auxiliary:
Nur Kinder können Feen sehen. (Aktiv)
Children can see fairies.
Feen können nur von Kindern gesehen werden. (Vorgangspassiv)
Fairies can only be seen by children.
German has another passive voice –Zustandspassiv– which is applicable to a large number of verbs that describe the result of an action. Sehen is not one of those verbs however, as it has no result, at least to German logic.
But fangen for example has.
Die Kinder fangen die Fee. (Aktiv)
The children catch the fairy.
Die Fee wurde von den Kindern gefangen. (Vorgangspassiv)
The fairy has been caught by the children.
Die Fee ist gefangen. (Zustandspassiv)
The fairy is caught.
As you can see, the use of the Zustandspassiv is straightforward to English speakers.
Because when you use können (in the sense of be able to), you always use the Vorgangspassiv. (Janka's answer nicely discusses the two Passivs in German.)
Thus, if you want to say "Fairies can be caught", you translate that as:
Feen können gefangen werden
Not as
Feen können gefangen sein
This isn't because of the verb fangen, which, as Janka's answer shows, is perfectly compatible with the Zustandspassiv, but because können in the sense of to be able only combines with the Vorgangspassiv.
This also means that whenever you see können with the Zustandspassiv, it has to mean something different. For example,
Die Fenster können geschlossen sein
doesn't mean that the windows are closable (it's not "Windows are able to be in the state of being closed"). It means that it's possible that right now, the windows are closed.
Because the infinitive is "von etwas/jemandem gesehen werden". The English equivalent would be "can be seen by someone/something" however, this is just another difference between these two languages.