Does anyone know in + den?
in+den means "into the" or "to the"
"in" is a two-way preposition which can take either Accusative or Dative. In this case it's in the accusative case which means there must be movement. Also the object must be masculine.
"Ich gehe in den Park" "I'm going to the park"
This is accusative as you are moving to the park.
If you were already in the park it would be dative - "Ich bin im (in+dem) Park"
Similarly for feminine and neuter nouns:
"Ich gehe in die Schule" "I go to school" vs "Ich bin in der Schule" "I'm in school"
"Ich gehe ins (in+das) Kino" "I'm going to the cinema" vs "Ich bin im (in+dem) Kino" "I'm in the cinema"
mojiradej, if I was you I would just learn off phrases as oppose to trying to understand the cases for your letter and postcard. That's what I'm doing and I've done well in the written expression part of the exam. Good luck.