nicht vs kein
German splits "not" into two different words depending on what's being negated: get the split right, and the rest of negation is just deciding where the word goes.
kein replaces "a": nicht negates everything else
If the noun you're negating would otherwise take ein (or no article at all), use kein. For verbs, adjectives, adverbs, and definite nouns, use nicht instead.
Ich habe keinen Hund.
Ich sehe den Hund nicht.
kein negates a noun by replacing its article; nicht just gets added, wherever the thing it negates sits.
kein Across Cases
kein is just ein with a "k" in front, so it declines exactly the same way, including the one accusative shift you already know.
| Masc. | Fem. | Neut. | Plural | |
|---|---|---|---|---|
| Nominative | kein | keine | kein | keine |
| Accusative | keinen | keine | kein | keine |
kein also covers the plural, unlike ein: Bücher (books) becomes keine Bücher (no books).
What nicht Negates
nicht attaches to whatever it's negating: a verb, an adjective, an adverb, or a noun that already has a definite article.
kein vs. nichts
Both can sound like "nothing" in English, but only one of them modifies a noun.
An article-like word that sits in front of a noun; it can't stand alone.
Ich habe kein Geld.
A pronoun that stands completely on its own, no noun needed.
Ich habe nichts.
Spot It in Real Sentences
Er kann ihn nicht trinken, denn es schmeckt nicht gut.
He can't drink it, because it doesn't taste good.
Otto isst das Brötchen nicht, denn er hat keinen Hunger.
Otto doesn't eat the roll, because he's not hungry.
Er hofft, dass er seine Katze und seinen Hund nicht vermisst.
He hopes that he doesn't miss his cat and his dog.
The Trap: kein on the Wrong Word
kein only ever negates a noun: never a verb or an adjective. Das ist kein gut is wrong, because gut is an adjective; it has to be Das ist nicht gut. Save kein for the moments you'd otherwise reach for ein; everywhere else, it's nicht.