Learn Danish: Han står og prøver et meget fugtgivende serum. - He is trying out a very moisturizing serum.

It is a compound adjective formed by combining the noun fugt (moisture) and givende, which is the present participle of the verb at give (to give). Literally meaning moisture-giving, it is the standard Danish way to say moisturizing.

Why doesn't fugtgivende get a -t ending to agree with the neuter noun et serum?

Adjectives formed from present participles (ending in -ende) are indeclinable in Danish. This means they never change their form to match gender or number, so fugtgivende stays exactly the same whether it describes an en word, an et word, or a plural noun.

Why does the sentence say he står og prøver instead of just prøver?

Danish uses position verbs like står, sidder, and ligger linked with og to create a continuous tense, similar to the English -ing form. Using står og prøver emphasizes that the action of trying the serum is ongoing right now while he happens to be standing.

Is serum an et word, and how would I make it plural?

Yes, serum is a neuter noun, so it takes et. Because it is a Latin loanword, it actually has two accepted plural forms in Danish: the traditional Danish serummer and the Latin sera.