Flyselskabet har et erstatningsfly.

Because Danish, like German, very often forms compound nouns by writing the parts together.

• fly = plane / aircraft
• selskab = company
• flyselskab = airline

Then -et is added to make it definite:

• flyselskab = an airline
• flyselskabet = the airline

So Flyselskabet is literally the airline.

It is the definite ending for a neuter noun in Danish.

In English, you usually put the before the noun.
In Danish, the definite article is often attached to the end of the noun instead.

• et flyselskab = an airline
• flyselskabet = the airline

This is very common in Danish:

• et hus = a house → huset = the house
• et fly = a plane → flyet = the plane

So -et here means the.

Because fly is a neuter noun in Danish, so it takes et.

Danish nouns have two grammatical genders:

• en-words (common gender)
• et-words (neuter)

Since fly is an et-word, compounds ending in fly are also treated as et-words:

• et fly
• et erstatningsfly

A useful rule is: in Danish compounds, the last part usually determines the gender.
Since the last part is fly, the whole word takes et.

That -s- is a linking element often used in Danish compounds.

Here:

• erstatning = replacement
• fly = plane
• erstatningsfly = replacement plane

The s does not usually have a separate meaning by itself. It just helps connect the two parts in a natural-sounding compound.

You will see this in many Danish compound nouns. Unfortunately, there is not always a simple rule for when the linking s appears, so it is often something you learn word by word.

Yes. Har is the present tense of at have = to have.

• jeg har = I have
• du har = you have
• han/hun har = he/she has
• flyselskabet har = the airline has

Unlike English, Danish does not change the present-tense verb depending on the subject. It stays har for all persons.

Because this is a simple main clause with normal subject–verb–object order:

• Flyselskabet = subject
• har = verb
• et erstatningsfly = object

So the structure is:

The airline + has + a replacement plane

That is very similar to English.

However, Danish is a V2 language, which means the verb must usually come in the second position in main clauses. If you move another element to the front, the verb still stays second:

• Flyselskabet har et erstatningsfly.
• I dag har flyselskabet et erstatningsfly. = Today the airline has a replacement plane.

So this sentence looks English-like, but Danish word order becomes more noticeably different in other examples.

Because Danish usually expresses the by adding an ending to the noun.

So instead of:

• English: the airline

Danish normally says:

• flyselskabet

This is called the suffixed definite article.

A separate word such as den or det is used in some other situations, especially when an adjective comes before the noun:

• flyselskabet = the airline
• det store flyselskab = the big airline

So in your sentence, no separate the is needed.

Because the two nouns are in different forms:

• Flyselskabet is definite = the airline
• et erstatningsfly is indefinite = a replacement plane

In Danish:

• indefinite singular uses en or et before the noun
• definite singular usually uses an ending on the noun

So:

• et flyselskab = an airline
• flyselskabet = the airline
• et erstatningsfly = a replacement plane
• erstatningsflyet = the replacement plane

No. In this sentence, fly means plane / aircraft.

That can be confusing for English speakers because fly looks like the English verb to fly.

Some useful distinctions:

• et fly = a plane
• at flyve = to fly
• en flyvning = a flight

So erstatningsfly is a replacement plane, not a replacement flight.

A rough English-friendly guide would be something like:

FLOO-sel-skah-beth har ed air-STAT-nings-floo

A few pronunciation notes:

• y in fly is not exactly like English oo or ee; it is a front rounded vowel that English does not really have.
• g in -selskabet is often soft or weakened in normal speech.
• et is often pronounced more like ed in connected speech.
• r in Danish is quite different from most English r sounds.
• Danish pronunciation is often much less phonetic than spelling suggests, so listening to native audio is especially important.

Yes. The exact English translation depends on context.

• replacement plane
• substitute plane
• backup plane
• spare aircraft

All of these can work in the right situation.

The Danish compound erstatningsfly literally suggests a plane used as a replacement for another one. In many learning contexts, replacement plane is the clearest direct translation.

The most likely plural forms are:

• et flyselskab = an airline
• flere flyselskaber = several airlines
• flyselskaberne = the airlines

And:

• et erstatningsfly = a replacement plane
• flere erstatningsfly = several replacement planes
• erstatningsflyene = the replacement planes

Notice that fly itself often has the same form in singular and plural:

• et fly = a plane
• flere fly = several planes

That same pattern often carries over into compounds ending in fly.