Learn Danish: Er der langt til stationen? - Is it far to the station?

In Danish, when talking generally about a distance, you use der (there) as the subject. If you ask Er det langt?, it means "Is it long?" referring to a specific physical object like a piece of string or a road. To ask if something is a long distance away, always use der.

Why does langt have a -t at the end?

The base word is lang (long or far). Here it is used as an adverb to describe the general situation of distance. In Danish, when you use an adjective in this abstract, adverbial way, you add a -t to the end.

How do I know whether to use -en or -et to say "the" for the word station?

stationen means "the station". A great rule of thumb for A2 learners is that almost all Danish nouns ending in -tion are common gender (en-words). That means they will reliably take -en on the end to say "the".