Breakdown of Он не заметил старой иконы за дверью.
не
not
он
he / it (for a masculine noun) / it (for a neuter noun)
дверь
door
старый
old
Questions & Answers about Он не заметил старой иконы за дверью.
Why is it старой иконы (genitive) instead of старую икону (accusative) since it is the direct object?
Because of the negated verb не заметил. In Russian, direct objects of negated verbs often take the genitive case instead of the accusative. This is known as the genitive of negation. It is especially common with verbs of perception, like noticing or seeing, and emphasizes that the object was entirely missed or absent from the subject's experience.
How does this vocabulary relate to our current lesson on nouns with dual plural forms?
While икона has a regular plural (иконы), its synonym образ is a classic example of a noun with dual plural meanings. The plural образа (stressed on the last syllable) strictly means religious icons, whereas образы (stressed on the first syllable) means images, forms, or literary characters. Note that if you used it in this exact sentence, you would need the genitive singular о́браза (не заметил старого о́браза), which looks identical to the special plural but carries a different stress.
Why is дверью in the instrumental case? Doesn't за sometimes take the accusative?
The preposition за takes the instrumental case when describing a static location, answering the question 'where?' (где?). Since the icon is simply located behind the door, we use the instrumental. If the sentence described motion directed behind the door, like someone hiding something behind the door (answering 'where to?' or куда?), you would use the accusative case (за дверь).