when do we use" den" and when do we use "det"? do they have any difference

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It’s based on gender. Because Danish is a common-neuter gender language. Our subjects are usually either common gender or neuter.
Common Gender uses “en” and neuter uses “et”.

“det” is used for “et” nouns and “den” is used for “en” nouns


Some more examples of how they can be used;

Et træ = a tree
Det træ = that tree

Et hus = a House
Det hus = that house

En lygtepæl = a lamppost
Den lygtepæl = that lamppost

En komiker = a Comedian
Den komiker = that Comedian

Sentence example;
Hvor gammel er den pige der? (Points to the girl)
- how old is that girl there?

Er det billede til salg?
- is that picture up for sale?

There is no definite rule to know when and why each subject is either gender. It is something where each word would need to be looked up in the dictionary or be memorized to find the gender.

Also, They both translate to “that” in English”

Hope this helps :)