Learn Danish: Er dit nye serum virkelig så velduftende? - Is your new serum really that fragrant?

It is a compound adjective made of the adverb vel (well) and the present participle duftende (smelling). We use the present participle, which ends in -ende, because the serum is actively doing the action of smelling good, rather than having an action done to it.

Why does nye have an -e on the end if serum is a singular word?

Even though serum is singular, it is preceded by the possessive pronoun dit. In Danish, possessive pronouns act like definite articles, which means any following adjective must take the definite, or weak, form by adding an -e.

How do we know the gender of the word serum from this sentence?

We can tell it is a neuter word (an et word) because it is preceded by the neuter possessive pronoun dit. If it were a common gender word (an en word), the sentence would use din instead.

Why doesn't the adverb virkelig end in a -t here?

When virkelig is used as an intensifying degree adverb before another adverb or adjective (like så), it is common and grammatically correct to drop the -t. While you might sometimes see virkeligt, leaving the -t off is completely standard in this context.