Learn Danish: Er det glat udenfor når det sner? - Is it slippery outside when it snows?

In Danish, just like in English, we need a subject to talk about the weather or general environmental conditions. Since there isn't a real person or object doing the action, we use a dummy subject. The neuter pronoun det (it) fills this role. We need it once for the condition (Er det glat) and once for the weather action (når det sner).

Since the dummy subject det is neuter, why doesn't the adjective glat get a -t ending?

You are right that adjectives describing the neuter dummy subject det must take their neuter form, which usually means adding a -t (for example, kold becomes koldt). But because glat (slippery) already ends in a -t in its base form, we do not add another one. The neuter form just stays glat.

Why does the first part swap the verb and subject (Er det) but the second part does not (det sner)?

The sentence begins with a question, which means we swap the verb and subject in the main clause (Er det...). However, når det sner is a subordinate clause because it starts with the conjunction når (when). In subordinate clauses, the normal order is kept where the subject (det) comes right before the verb (sner), even if the overall sentence is a question.

Why is the word for when når here, instead of da?