Questions & Answers about Das Fenster ist geöffnet.
Because Fenster is a neuter noun in German, and the singular nominative article for neuter nouns is das.
- der = masculine
- die = feminine
- das = neuter
So the dictionary form is das Fenster.
Note that die Fenster does exist, but that would be plural: the windows.
It is in the nominative case because it is the subject of the sentence.
A quick way to see that:
- What is opened / is open? → Das Fenster
So das Fenster is the thing the sentence is about, which makes it the subject.