Breakdown of Sie prüft das System eines Autos.
ein
a, an (eine)
sie
she
das Auto
the car
prüfen
to check / to test
Questions & Answers about Sie prüft das System eines Autos.
Why does ein Auto change to eines Autos?
This sentence uses the Genitive case to show possession or association, like saying 'the system of a car'. Because Auto is a neuter noun, its indefinite article ein changes to eines, and we add an -s to the noun itself.
I learned that masculine and neuter nouns often add -es in the Genitive. Why is it just Autos here?
You are right that many masculine and neuter nouns take an -es ending (like des Jahres). However, nouns that end in a vowel, like Auto, just take an -s. Adding -es would sound awkward to pronounce.
In English, we say 'a car's system'. Can I put the Genitive first and say Sie prüft eines Autos System?
No, you should stick to putting it after the noun. In German, the Genitive phrase almost always comes directly after the thing being owned. It is much easier to translate it in your head as 'the system of a car' (das System eines Autos).
How do we know Sie means 'She' here, and not 'They' or the formal 'You'?
You can tell by looking at the verb! If Sie meant 'They' or the formal 'You', the verb would take the plural ending, making it Sie prüfen. Because it ends in a -t (prüft), it has to be the singular 'She'.