Everything you need to know is on the Interrail.eu website. Travel in your own country | Interrail.eu

Keep in mind that the outbound trip to leave your country of residence doesn’t have to come before your inbound journey depending on how you planned your trip.

Yes I read that.. but what if I’m just travelling trough my country one day and the other day I‘m coming back? So from Sweden trough Germany to the Netherlands and then from there back to Germany.

It’s not possible. Because 1 Inbound Trip (from Denmark to Germany and (the same day?) to The Netherlands and than back to Germany (second inbound trip). This is not possible because you are comming 2 times back to Germany. You could pay a ticket (online not available) at DB or NS from the “Grenztarifpunkt” to your home city in Germany.

Or you could try to go with the IC from Denmark to Flensburg/Hamburg. You have to pay a Ticket (online not possible) from Flensburg(gr)* to Flensburg or when the train does not stop in Flensburg a ticket Flensburg(gr)* to Hamburg and you start your outbound trip from Hamburg to the Netherlands. You write than in your travel pass or mobile pass that you are doing your outbound journey from Flensburg/Hamburg to Den Haag. 

*(gr) stands for Grenze = border