Breakdown of Ein Tropfen des Zaubertranks heilt die Wunde.
ein
a, an (eine)
die Wunde
the wound
der
who, which, that (relative pronoun; die, das)
der Tropfen
the drop
Questions & Answers about Ein Tropfen des Zaubertranks heilt die Wunde.
Why does this sentence use the genitive des Zaubertranks instead of just the partitive apposition (ein Tropfen Zaubertrank)?
Because the potion is specific. We use the partitive genitive when the substance is accompanied by an article, adjective, or pronoun (here, des translates to "of the"). If we were talking about an unspecified substance ("a drop of magic potion"), we would use the partitive apposition without an article or genitive ending: Ein Tropfen Zaubertrank.
Why is there an -s at the end of Zaubertranks?
It is the standard genitive ending. Der Zaubertrank is a masculine compound noun (derived from der Trank). Masculine and neuter nouns almost always add an -s or -es when they are in the genitive case.
The verb heilen has a direct object here (die Wunde). Can it also be used without one, like in English?
Yes. Heilen (to heal) can be used transitively with an accusative object, as in this sentence where the drop heals the wound. It can also be used intransitively to describe the healing process itself, for example: Die Wunde heilt schnell (The wound is healing quickly).
If ein Tropfen is "a drop", how would you say "two drops"?
You would say zwei Tropfen. Der Tropfen is a masculine noun ending in -en, and like most nouns with this ending, its form does not change in the plural. You can only tell if it is plural by the article, a number, or the verb conjugation in the sentence.