This is the 587th most frequent Icelandic word.
opnar
"Opnar" means "opens" in English.
In this sentence, the word "opnar" is used as a verb in the third person singular present tense, indicating the action of opening something, which refers to the door here.
Hún opnar dyrnar.
She opens the door.
Here, "opnar" is again a verb in the third person singular present tense, describing the capability of the key to unlock and open the cabinets.
Þessi lykill opnar alla skápana.
This key opens all the cabinets.
In this example, "opnar" denotes an autonomous action performed by the machine and is used as a verb in its indicative present form.