Learn Danish: Jeg ved ikke om dette ridsede spejl er dyrt. - I do not know whether this scratched mirror is expensive.

ridsede is the past participle of at ridse (to scratch) functioning as an adjective. Because it follows the demonstrative pronoun dette (this), it must be in its definite form. For past participles that end in -t in their base form (like ridset), you add an -e to make them definite, resulting in ridsede.

Since "dette" means "this", why isn't the noun in the definite form "spejlet"?

In Danish, when you use a demonstrative word like dette (this) or denne (this), the noun that follows stays in its basic, indefinite form (spejl). The adjective still gets the definite -e ending, but the noun itself does not take the -et suffix.

Why use "om" instead of "hvis"?

You use om when expressing uncertainty or a question with multiple possibilities, exactly like "whether" in English. You only use hvis for conditional statements, like "if it rains, I will stay inside."

Why is there a "-t" at the end of "dyrt"?

dyrt is an adjective describing spejl (mirror). Because spejl is a neuter noun (an et-word), the base adjective dyr (expensive) must add a -t to agree with it.