"It is not her."
Translation:Det er ikke hende.
When the meaning is abstract (We don't know exactly what "it" is until we identify that it's not her) we use det in Danish, den would normally follow once the object has been established. For example:
A: Hvad er det? (What is that?)
B: Det er en bil (It is a car)
A: Nåh, hvorfor er den låst? (Why is it locked?)
B: Det ved jeg ikke (I don't know [also another example where to use det instead of den])
Languages differ over whether they require the subject or object form of pronouns when they are the complements of the verb to be.
I note that Danish is using the object form here, like English ("it is not me/her"), French (ce n'est pas moi/elle) or Norwegian (det er ikke meg/henne). Spanish would use the subject form (no soy yo / no es ella). Swedish seems to be able to use either, with the object form mig/henne being more informal than the subject form jag/hon. English has the option of using the subject form ("it is not I/she") in very formal or archaic usage. Is Danish firmly on the object side?
Using "hendes" instead of "hende" here would make the sentence mean "It is not hers", meaning the thing does not belong to her. "Hendes" is a possessive pronoun, which English confusingly uses "her" for when in front of a noun.
"Her book" translates to "hendes bog".
"The book is for her" translates to "bogen er til hende"