Learn Danish: Mandens historie er simpelthen hjerteskærende. - The man's story is simply heartbreaking.

This is a great example of object incorporation. It combines the noun hjerte (heart) with skærende, which is the present participle (the -ende form) of the verb at skære (to cut). Instead of writing it out as an action happening to an object, Danish rolls the object directly into the participle to create the single compound adjective hjerteskærende.

Will hjerteskærende change if the subject becomes plural, like historierne (the stories)?

No, it will stay exactly the same. Unlike regular adjectives, present participles ending in -ende are indeclinable. This means they do not add an -e or a -t to match the gender or number of the noun they describe. It is always just hjerteskærende.

What role does simpelthen play here, and why is it placed after the verb?

Simpelthen means "simply" or "absolutely" and is used as an intensifier to add emotion to the statement. It acts as a central adverb (like ikke, ofte, or altid), which means that in a main clause like this one, it must be placed directly after the finite verb (er).

Could I say historien af manden instead of mandens historie?