This is the 925th most frequent Icelandic word.
opnað
"Opnað" means "opened" in English.
The word 'opnað' is used here as the past participle form of the verb 'opna', following the auxiliary verb 'hef', to indicate a completed action in the perfect tense.
Ég hef opnað gluggann.
I have opened the window.
Here, the verb 'opna' is in its infinitive form, following the modal verb 'skulum', to suggest a proposal to perform the action of opening.
Við skulum opna dyrnar.
Let us open the doors.
In this sentence, 'opnuð' (a derived form of 'opnað') is used in the context of a passive voice construction to describe the book being in an opened state caused by the teacher.