Learn Russian: Собака дрожала от холода, а не от страха. - The dog was shivering from cold, and not from fear.

The preposition от followed by the Genitive case is used for involuntary physical or emotional causes. The dog didn't choose to be cold or afraid; these are uncontrollable reactions causing the shivering. If an action is driven by a conscious motive (like doing something 'out of curiosity' or 'out of spite'), you would use из instead.

Why use а не instead of но не or и не for 'and not'?

The conjunction а is used to show a direct contrast between two elements, effectively replacing one idea with another ('A, not B'). When you are setting up a direct opposition between two nouns like this, а не is the standard structure. Но (but) is typically used to introduce a contradiction or an unexpected twist in a full clause.

Why is the verb дрожала in the imperfective aspect?

Shivering is a continuous, ongoing physical state rather than a single, completed event. Because the sentence describes an ongoing process that the dog was experiencing in the past, the imperfective aspect is required.

What case do холода and страха take, and how is it formed here?