Nicht or kein?
Use kein to negate a noun with ein or no article. Kein has the same endings as ein. Use nicht for verbs, adjectives, or a specific phrase. Nicht usually comes before the element it negates.
Understand the idea
Kein negates a noun that would otherwise have ein or no article. Nicht negates a statement, an adjective, an adverb or a particular part of a sentence. The choice depends on what you are denying, not simply on the English word not.
Build it step by step
If the positive noun phrase is ein Auto, its negative counterpart is kein Auto. Kein follows ein-type endings and has plural forms such as keine. With a predicate adjective, place nicht before the adjective. When negating a whole sentence, nicht often appears later in the clause, but before a final infinitive or participle; focused negation goes directly before the part being contrasted.
See it in a sentence
Ich habe kein Auto.
Der Kaffee ist nicht heiß.
A useful memory aid
Build the positive sentence first, then identify exactly what is being denied. This makes the choice between kein and nicht much clearer.
A common trap
Avoid kein before a predicate adjective. The room is not small uses nicht klein, while no room uses kein Zimmer.