“Nicht” or “kein”: which word says no, and where you put it.

Ich habe nicht ein Auto and ich trinke kein den Kaffee are the two ways this one goes wrong. The first negates with the word that negates nearly everything, in the one place where German keeps a separate word for the job. The second reaches for that separate word and then leaves standing the article it came to replace. Neither sentence is short of vocabulary: both words are known, and what is missing is which of them the sentence had asked for.

Kein goes exactly where ein would have gone, whenever the noun is the thing you are denying, and it declines as ein does, plus a plural where ein has none: kein Auto, keine Zeit, keinen Kaffee, keinem Kind, keine Kinder. It also covers the noun that had no article to begin with, because an indefinite plural and an uncountable thing take no indefinite article in German: ich habe keine Zeit, ich trinke keinen Kaffee, wir haben keine Kinder. That is why kein den Kaffee cannot exist. Kein has taken the article’s place, so there is no room left for an article behind it.

Nicht negates everything else. A verb, an adjective, an adverb, a name, a prepositional phrase, and a noun that already carries a definite article or a possessive: ich trinke den Kaffee nicht, das ist nicht mein Auto, das ist nicht Peter, der Kaffee ist nicht heiß.

Then the half that decides more than which of the two words you reach for. Nicht has a position in the sentence, and the position is not a matter of taste. When you are denying the whole sentence, nicht goes late, immediately in front of whatever closes the clause, which is where the clause-final verb from the post on wenn and als pays off: ich habe das Buch nicht gelesen, ich kann heute nicht kommen, and ich fahre nicht nach Berlin, where the destination belongs to the verb and closes the clause as a participle would. When you are denying one element and putting something else in its place, nicht goes directly in front of that element: ich fahre nicht nach Berlin, sondern nach Hamburg leaves the travelling alone and replaces the destination.

Which case those phrases take is a separate question from whether they are negated, and nach Berlin keeps the form it had in both sentences. The post on in den and in dem is the one about case after a preposition.

The edge that trips people up

The short reflex, no ein means kein, holds as long as the noun is the thing you are denying, and that condition is doing real work. Ich trinke keinen Kaffee denies coffee: not a drinker. Ich trinke den Kaffee nicht denies the drinking of one particular cup: it is in front of you and you are leaving it. Both sentences are correct German and they are correct about different things, so this is not a pair where one is the safer choice.

There is also the sentence where nicht stands in front of a noun with ein, which the first rule seemed to forbid. It happens when a correction is following: you are not denying that there is one of the thing, you are replacing the number or the word with another. Das ist nicht ein Problem, sondern zwei. The sondern is the giveaway, and without something like it the sentence goes back to kein.

Nein belongs to neither of these. It answers a question, standing outside the sentence that follows it, and it negates nothing inside that sentence: Trinkst du Kaffee? Nein, ich trinke keinen Kaffee. Saying nein has not saved you the kein, and the two words look related without doing related work.

How to hear it

The first thing to settle is which word you are actually denying, and the second is what stands in front of it.

If what stands in front of it is ein, or nothing at all, you want kein, and then it takes the ending ein would have taken in that case and gender. If what stands in front of it is der, mein or dieser, or if the thing you are denying is a verb, an adjective, an adverb or a bare place name (nach Berlin, zu Hause), you want nicht. A preposition on its own decides nothing: mit keinem Kollegen is the ordinary German for denying the noun inside a prepositional phrase. Then one question is left over: where does it go? In front of the element, if you are denying that element and putting something else in its place. Late in the clause, in front of whatever closes it, if the whole sentence is what you are denying, which is the ordinary case.

That is not a complete account of where nicht lands in a long sentence, because what is already known between you and the person listening pulls it about as well. What it gives you is two places to try, and a reason to prefer one of them.

Ich habe das Buch nicht gelesen and ich habe nicht das Buch gelesen are both German, and each of them says something different: the first says you have not read it, the second says the book is not what you read. No word was added or taken away between them. Nicht moved one place to the left, and that is the whole of the difference between the two reports.