When do you put an e at the end of mein in a German sentence?

You never put an "e" at the end of "mein" in a German sentence.

"Mein" is the nominative and accusative form of the possessive adjective "my" when referring to masculine singular nouns. It never takes an ending, regardless of its position in the sentence.

Here are some examples:

* Mein Auto ist rot. (My car is red.)

* Ich habe mein Buch vergessen. (I forgot my book.)

If you are referring to a feminine singular noun, you would use "meine". If you are referring to a plural noun, you would use "meine" for both masculine and feminine.

* Meine Katze ist schwarz. (My cat is black.)

* Meine Bücher sind auf dem Tisch. (My books are on the table.)