Skadedyret kommer frem om natten.

Because -et is the definite ending for a singular neuter noun in Danish.

• et skadedyr = a pest
• skadedyret = the pest

Danish usually adds the to the end of the noun instead of using a separate word in front of it.

The base noun is dyr (animal), which is neuter: et dyr.
Since skadedyr is a compound built on that noun, it is also neuter:

• et skadedyr
• skadedyret

So the definite singular ending is -et.

Skadedyr literally means something like harmful animal:

• skade = damage, harm
• dyr = animal

In actual usage, it usually means pest. It can refer to insects, rodents, or other animals that cause damage.

Kommer frem means comes out, emerges, or appears.

The verb is:

• komme = to come
• frem = forward, forth, into view

Together, komme frem often means that something becomes visible or appears from hiding. In this sentence, it fits the idea of a pest coming out at night.

Sometimes, yes, but the nuance is a little different.

• komme frem = come out into view, emerge, appear
• komme ud = come out, come outside, exit

In this sentence, kommer frem suggests that the pest becomes active or visible.
Kommer ud would sound more like it physically comes out from a place.

So kommer frem is a very natural choice here.

Kommer is the present tense of komme (to come).

• jeg kommer = I come / I am coming
• skadedyret kommer frem = the pest comes out

In Danish, the present tense is often used for general truths or habitual actions, just like in English:

• The pest comes out at night

This is a very common Danish time expression.

• om dagen = during the day / by day
• om natten = at night
• om morgenen = in the morning
• om aftenen = in the evening

Here, om is used for something that happens during a recurring part of the day.

So om natten is the normal idiomatic way to say at night.

That is just how this common time expression works in Danish.
With parts of the day, Danish often uses om + definite form:

• om morgenen
• om eftermiddagen
• om aftenen
• om natten

Even though English says at night without the, Danish uses the definite noun here.

Usually no, not in this meaning.

• om natten = at night, as a general or habitual time expression
• i natten is not the normal way to express that idea

If you want to say that something happens regularly or characteristically at night, om natten is the natural phrase.

The basic structure is:

• Skadedyret = subject
• kommer = verb
• frem = particle/adverb
• om natten = time expression

So:

Skadedyret kommer frem om natten.
= The pest comes out at night.

This is normal Danish main-clause word order: subject + verb + other elements.

Yes. In a Danish main clause, the finite verb usually stays in second position.

So you can say:

Om natten kommer skadedyret frem.

Notice that after the fronted time phrase Om natten, the verb kommer comes before the subject skadedyret.

That is a very important Danish pattern.

A rough guide is:

SKA-the-dy-rət

A few helpful points:

• sk before a sounds like regular sk
• dy has a Danish y sound, which is not exactly like English oo or ee
• the r in Danish is softer than in English
• the ending -et in definite neuter nouns is often pronounced like a weak -əd or -et, depending on accent and speech style

If you are learning pronunciation, dyr and the Danish y sound are worth practicing separately.

Grammatically, skadedyret means the pest, so it is singular and definite.

But in real usage, Danish can sometimes use a singular definite noun to talk about a type of creature in a general way, depending on context. So this sentence could mean:

• one specific pest that has already been mentioned, or
• a pest species/type being described generally

The surrounding context would tell you which meaning is intended.