Questions & Answers about Es ist keine Kartoffel, sondern eine Nuss.
Why do we use sondern here instead of aber for "but"?
In German, you use sondern instead of aber when you are correcting a negative statement. Because the first part says it is not a potato (keine Kartoffel), we use sondern to introduce what it actually is (a nut).
Why is it keine Kartoffel instead of nicht eine Kartoffel?
When you negate a noun that has an indefinite article (like "a" or "an") or no article at all, you almost always use a form of kein instead of nicht. Since it would be eine Kartoffel if it were positive, it becomes keine Kartoffel when negated.
Why do keine and eine both end in -e?
Both Kartoffel (potato) and Nuss (nut) are feminine nouns. We are using the nominative case here because we are simply stating what something is, so the articles take the feminine -e ending.
Where is the verb in the second part of the sentence?
The second half is actually a shortened version of sondern es ist eine Nuss. Because the subject and verb (es ist) are exactly the same as in the first half of the sentence, we can leave them out to sound more natural.