Mellemlandingen er kort.

Because Danish usually expresses the by adding it to the end of the noun.

• en mellemlanding = a layover / a stopover
• mellemlandingen = the layover / the stopover

The ending -en is the definite singular ending for a common-gender noun.

In Danish, a simple definite noun normally takes the article as a suffix, not as a separate word.

So Danish prefers:

• mellemlandingen = the layover

not usually:

• den mellemlanding

You normally use separate den/det/de when there is an adjective or another modifier:

• den korte mellemlanding = the short layover

So:

• mellemlandingen = the layover
• den korte mellemlanding = the short layover

Yes. Danish, like German, often writes compound nouns as one word.

mellemlanding is made from:

• mellem = between
• landing = landing

So the literal idea is something like between-landing, which gives the sense of a stopover / layover.

It is a common-gender noun, also called an en-word.

You can see that from:

• en mellemlanding = indefinite singular
• mellemlandingen = definite singular

If it were a neuter noun, you would expect et in the indefinite singular instead.

Er is the present tense of at være = to be.

So:

• er = is / are

In this sentence, it simply means is:

• Mellemlandingen er kort. = The layover is short.

Useful related forms:

• at være = to be
• er = is / are
• var = was / were
• været = been

Here kort is a predicative adjective: it comes after er and describes the subject.

In Danish, predicative adjectives agree with the noun, but with kort the singular forms happen to look the same.

Compare:

• en kort mellemlanding = a short layover
• mellemlandingen er kort = the layover is short
• korte mellemlandinger = short layovers
• mellemlandingerne er korte = the layovers are short

So korte is used in plural and in many attributive definite contexts, while kort is correct here.

It means short in duration here.

The adjective kort can refer to both:

• physical length: a short road
• time: a short meeting

With mellemlanding, the natural interpretation is short in time.

In this sentence, yes. It is:

• subject: Mellemlandingen
• verb: er
• adjective/complement: kort

So it matches English The layover is short quite closely.

But Danish has an important rule in main clauses: the finite verb is usually in second position. For example:

• I dag er mellemlandingen kort. = Today the layover is short.

Even when another element comes first, er still stays in second position.

The plural forms are:

• mellemlandinger = layovers / stopovers
• mellemlandingerne = the layovers / the stopovers

Example:

• Mellemlandingerne er korte. = The layovers are short.

This is useful because it also shows the plural adjective form korte.

You should look up the base noun form: mellemlanding.

Dictionaries usually list nouns in their indefinite singular form, often with the article:

• en mellemlanding

So if you see mellemlandingen, remove the definite ending -en and search for mellemlanding.