Breakdown of Ich kaufe ein Mückenspray. Brauchst du auch eins?
ich
I
ein
a, an (eine)
du
you (informal singular)
auch
also, too
Questions & Answers about Ich kaufe ein Mückenspray. Brauchst du auch eins?
Why is it eins at the end instead of just ein?
In the first sentence, ein is an article directly attached to the noun Mückenspray. But in the second sentence, the word stands alone to mean 'one' and replaces the noun entirely. When an indefinite article stands alone as a pronoun for a neuter noun like das Mückenspray, it adds an -s ending, becoming eins.
What gender and case is eins in this sentence?
It is neuter and in the accusative case. It is neuter because it refers back to das Mückenspray. It is in the accusative case because it is the direct object of the verb brauchen (what do you need? -> one). For neuter nouns, the standalone pronoun is eins in both the nominative (subject) and accusative (object) cases.
How do I know the gender of a compound word like Mückenspray?
In German, the last word in a compound noun always determines its gender. Since das Spray is neuter, das Mückenspray is also neuter. The first part, Mücken (mosquitoes), is plural, but that does not affect the gender of the whole combined word.
If I wanted to reply 'I do not need one', how would I say that?
You would use the negative pronoun keins. The reply would be: Ich brauche keins. Just like eins, it takes the -s ending because it replaces a neuter noun in the accusative case.