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