”Den idén” means ”that idé” (previously mentioned) whereas only ”idén” means ”the idea”.

• idén the idea

• den idén that idea

• den där idén that idea

• den här idén (or, denna idé) this idea

The difference between den idén and den där idén is a little bit nuanced, so just ignore it for the moment.

Forward would be framåt. Fram actually means forth. So, framtid literally means forth-time.