Demonstrative pronouns specify objects as well as distance to the objects.
Denne and den is used with singular masculine and feminine nouns.
Dette and det is used with singular neuter nouns.
Disse and de is used with plural nouns.
Note that the nouns following the demonstratives take the definite form: denne bilen etc.
Close
Remote
Masculine
denne koppen = this cup
den koppen = that cup
Feminine
denne døra = this door
den døra = that door
Neuter
dette glasset = this glass
det glasset = that glass
Plural
disse koppene = these cups
de koppene = those cups
You choose the correct form of the pronoun based on the distance between yourself and the object you are talking about, whether it’s close to you or remote from you.
Mind, however, that everything is relative.
If you, for example, were speaking about something close to you, then you would use denne/dette or disse, but at the same time the same objects can be remote from your interlocutor, the other person in your conversation. So he or she would probably use den/det or de.
Let’s look at some examples.
Here you have a small dialogue between me and an imaginary interlocutor (the other person in my conversation):
A: Jeg liker disse blomstene.
I like these flowers (the flowers are close to me)
B:De?
Those? (the same flowers, but remote from my interlocutor’s perspective)
A: Ja, disse her.
Yes, these ones here (near me)
B: Jeg foretrekker de der.
I prefer those (remote from my interlocutor’s perspective, but still near mine)