Breakdown of Das ist nicht dein Kaffee, das ist meiner.
sein
to be
der Kaffee
the coffee
nicht
not
dein
your (informal)
Questions & Answers about Das ist nicht dein Kaffee, das ist meiner.
Why does this sentence use meiner instead of just mein?
Because it is acting as a standalone pronoun replacing a noun. If you put the noun after it, you use the standard possessive word: mein Kaffee (my coffee). But when you drop the noun and just say "mine", the word has to show the gender of the missing noun. Since Kaffee is masculine and is the subject here (nominative case), it takes the -er ending to become meiner.
If Kaffee is masculine and nominative, why doesn't dein have an -er ending too?
Because dein is used directly in front of the noun here, acting like an article. In the nominative case, masculine possessive articles like mein and dein don't get an extra ending. The -er ending only appears when the possessive word completely replaces the noun rather than sitting next to it.
Could I use meins instead of meiner here?
Grammatically, no. meins is the neuter pronoun. You must use meiner because it refers back to der Kaffee (masculine). However, if you were talking about a neuter noun, like das Wasser, you would say: Das ist nicht dein Wasser, das ist meins.
Why do we use Das at the beginning of both phrases? Shouldn't it be Der, since Kaffee is masculine?