I study German. I don't know when to write dein or deine. For example: Dein Buch and Deine Sporttasche.

  • Hi and welcome to German Language Stack Exchange. Feel free to take a tour of the site. Visit the help center to learn more about how it works. Your question boils down to ‘how do I recognise a noun’s gender?’ – Jan Sep 22 '16 at 20:52

Dein is a Possesivartikel. It depends on what you are talking about.

For the case Nominativ (what you're talking about) there are the following rules:

  • Maskulinum (der) -> dein
  • Femininum (die) -> deine
  • Neutrum (das) -> dein
  • Plural (die) -> deine

So regarding your examples:

Das Buch (Neutrum) -> Dein Buch

Die Sporttasche (Femininum) -> Deine Sporttasche