Breakdown of Er isst keine Kartoffel, sondern eine Nuss.
ein
a, an (eine)
er
he
kein
no, not any
essen
to eat
Questions & Answers about Er isst keine Kartoffel, sondern eine Nuss.
Why use sondern instead of aber since they both mean "but"?
sondern is used specifically to correct a negative statement. Because the first part says he is eating keine Kartoffel (no potato), we use sondern to mean "not this, but rather that." If you were just adding a contrast without correcting a negative, you would use aber.
Why does the sentence use keine Kartoffel instead of nicht eine Kartoffel?
In German, when you want to negate a noun that has "a" or "an" in front of it (like a potato), you almost always use the negative article kein rather than nicht ein. Since Kartoffel is a feminine noun, it takes the feminine ending and becomes keine Kartoffel.
Why don't we need to repeat the verb after sondern?
Just like in English, if the verb is exactly the same for both parts of the sentence, you can drop it in the second half to avoid repeating yourself. The phrase sondern eine Nuss is simply short for sondern er isst eine Nuss.
Is the comma before sondern strictly necessary?
Yes! In German, you must always put a comma before contrasting conjunctions like sondern and aber, even if they are only connecting short phrases and not full sentences.