I timen forklarer læreren, at "hus" er et navneord, og at "løber" er et udsagnsord.

I timen means during the lesson or in class.

Danish often puts a time expression first when it sets the scene for the sentence. So instead of starting with the subject (læreren = the teacher), the sentence starts with the context: I timen.

This is very natural in Danish:

• I timen forklarer læreren ...
• Om morgenen drikker jeg kaffe.
• I går så vi en film.

So I timen is not the subject. It is just a time phrase placed first.

This is because Danish follows the V2 rule in main clauses.

V2 means the finite verb normally comes in the second position in the sentence.

If the subject comes first:

• Læreren forklarer ... = The teacher explains ...

But if another element comes first, such as a time phrase:

• I timen forklarer læreren ...

Here the first position is taken by I timen, so the verb forklarer must come second, and the subject læreren comes after it.

This word order is very important in Danish main clauses.

The -en ending marks the definite singular for many common nouns in Danish.

So:

• en time = a lesson / an hour
• timen = the lesson

and:

• en lærer = a teacher
• læreren = the teacher

Unlike English, Danish usually adds the definite article to the end of the noun instead of putting a separate word before it.

Compare:

• English: the teacher
• Danish: læreren

Because navneord and udsagnsord are neuter nouns in Danish, so they take et in the singular indefinite form.

• et navneord = a noun
• et udsagnsord = a verb

In Danish, nouns are either common gender or neuter:

• common gender: en
• neuter: et

You usually have to learn the gender together with the noun.

Because the sentence is classifying the words.

When you say that something belongs to a category, Danish usually uses the indefinite form:

• Han er læge. = He is a doctor.
• Det er en bil. = It is a car.
• Hus er et navneord. = House is a noun.

Here the idea is not “the noun” as one specific noun already known, but “a noun / a verb” as a category.

Here at means that, not to.

Danish at can have two different jobs:

• at løbe = to run → infinitive marker
• Jeg ved, at han kommer. = I know that he is coming. → conjunction

In your sentence, at introduces a clause after forklarer:

• forklarer læreren, at ... = the teacher explains that ...

So this at is a conjunction meaning that.

Because there are two parallel clauses:

• at hus er et navneord
• og at løber er et udsagnsord

Repeating at makes the structure clear and balanced. This is very natural in Danish.

You may sometimes hear or see the second at left out in some contexts, but in careful standard Danish, repeating it here is completely normal and often preferable.

So:

• ..., at X, og at Y.
  is a very good pattern.

Because the sentence is mentioning the words, not using them for their ordinary meaning.

• hus is being discussed as a word
• løber is being discussed as a word

In English, compare:

• House is a noun.
• Run is a verb.

You are not talking about an actual house or an actual act of running. You are talking about the word itself.

That is why Danish often puts such words in quotation marks. In explanations, bold type can do the same job:

• hus
• løber

Yes, the basic dictionary form is løbe (to run), but løber is still a verb form.

• løbe = infinitive
• løber = present tense

So even though løbe is the form you would usually find as the headword in a dictionary, løber still belongs to the word class verb.

English does the same:

• run is a verb
• runs is also a verb
• running is also a verb form

So the sentence is perfectly fine: løber is indeed et udsagnsord.

They are very common traditional school grammar terms in Danish:

• navneord = noun
• udsagnsord = verb

You may also see more Latin-based terms:

• substantiv = noun
• verbum = verb

Both sets are understood, but navneord and udsagnsord are especially common in school contexts and when explaining grammar to children.

So the sentence sounds very natural in a classroom setting.

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

• I timen = during the lesson / in the lesson period
• I klassen = in the classroom or sometimes in class

So:

• I timen focuses more on the lesson as an activity or time period.
• I klassen focuses more on the class/classroom setting.

In this sentence, I timen is a very natural choice because the teacher is explaining grammar during the lesson.