Questions & Answers about Деревья зимами покрываются белым снегом.
Why do we use the plural instrumental зимами here instead of the singular зимой?
The instrumental plural of a season shows repeated action over time. While зимой just means in winter, зимами emphasizes during winters or every winter.
Wait, we have two phrases in the instrumental case here (зимами and белым снегом). Do they do the same thing?
No, they have entirely different roles! зимами is the instrumental of time, showing when this happens. белым снегом is the instrumental of means, required by the verb покрываться to show what the trees are being covered with.
Why use the reflexive verb покрываются instead of just покрывают?
The verb покрывать is active and means to cover (e.g., they cover the table). Adding the reflexive ending -ся makes it passive or intransitive here, meaning to become covered. So, the trees aren't covering anything; they are becoming covered.
I sometimes forget the plural of дерево. Does it follow a pattern?
It uses a special irregular plural stem with a soft sign: деревья (Nominative), деревьев (Genitive), деревьям (Dative). It belongs to a group of nouns that do this, just like брат (братья) and стул (стулья).