Learn Russian: Волонтёры набрали дров для костра. - The volunteers gathered a lot of firewood for the bonfire.

This is exactly what the prefix на- does in this lesson's topic! When на- is added to a verb to mean accumulating a quantity of something (like набрать), the object usually takes the genitive case. It acts like a partitive genitive, meaning they gathered a quantity of firewood rather than specific, definite logs.

Is дров a singular or plural word?

It is the genitive plural of дрова. In Russian, firewood is almost always treated as a plural-only noun, so you will see it declined in the plural forms. The genitive plural of дрова drops the -а ending, leaving the zero ending дров.

What is the nominative form of костра, and why does it lose a letter here?

The nominative is костёр (bonfire). The ё is a fleeting vowel. When you add endings to the word, such as the genitive -а required by the preposition для, the ё drops out completely, making it костра rather than the incorrect костёра.

What is the difference between using набрали and собрали for gathering firewood?

Собрали simply means gathered or collected together in a general sense. By using набрали, the sentence specifically emphasizes the accumulation of a mass or quantity. It implies they gathered a whole bunch of firewood, likely until they had enough for their purpose.