Min lærer vil forklare sammenligningen, når timen begynder.

Because lærer is a common gender noun in Danish, and min is the possessive form used with common gender singular nouns.

• en lærer = a teacher
• min lærer = my teacher

You would use mit with a neuter singular noun:

• et hus = a house
• mit hus = my house

So the gender of the noun decides whether you use min or mit.

Here, vil means will in the sense of future time: will explain.

So:

• vil forklare = will explain

However, Danish vil can also sometimes mean wants to, depending on context. For example:

• Jeg vil spise can mean I want to eat

In your sentence, though, Min lærer vil forklare ... is most naturally understood as a future statement: My teacher will explain ...

Because after a modal verb like vil, Danish normally uses the infinitive without at.

So:

• vil forklare = will explain
• kan komme = can come
• skal læse = must/is going to read

You would not normally say vil at forklare.

This is similar to English, where we say will explain, not will to explain.

Sammenligningen means the comparison.

In Danish, the definite article is usually attached to the end of the noun:

• en sammenligning = a comparison
• sammenligningen = the comparison

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

Why definite here? Usually because the speaker has a specific comparison in mind, or because it is understood from context.

The base form is sammenligning.

Here is the pattern:

• en sammenligning = a comparison
• sammenligningen = the comparison

It comes from the verb at sammenligne, meaning to compare.

So:

• at sammenligne = to compare
• en sammenligning = a comparison

Because når timen begynder is a subordinate clause, and Danish normally uses a comma to separate clauses.

So the sentence is:

• Main clause: Min lærer vil forklare sammenligningen
• Subordinate clause: når timen begynder

Danish comma usage is often more visible than in English, so learners notice it quickly.

Because in a Danish subordinate clause, the word order is usually subject + verb, not the verb-second pattern of a main clause.

Compare:

• Main clause: Timen begynder = The lesson begins
• Subordinate clause: når timen begynder = when the lesson begins

This is an important difference in Danish:

• Main clauses often follow verb-second word order
• Subordinate clauses usually do not

So after når, timen comes before begynder.

Here, når means when.

So:

• når timen begynder = when the lesson begins

A useful point for English speakers: Danish når is used for when in situations that are habitual or expected in the future.

Examples:

• Når jeg kommer hjem, spiser jeg = When I get home, I eat / I’ll eat when I get home

So in your sentence, it introduces the time clause.

Because Danish often uses the present tense for future events, especially in time clauses.

So:

• timen begynder literally looks like the lesson begins
• but in context it means when the lesson begins / when the lesson starts

This is very normal in Danish, and English does something similar:

• I’ll call you when the lesson starts not usually when the lesson will start

So the Danish structure is actually quite close to English here.

It can mean either, depending on context, but here it means the lesson or the class period.

• en time can mean an hour
• en time can also mean a lesson/class period

So:

• timen begynder here is understood as the lesson begins or class starts

Context tells you which meaning is intended.

The sentence has two parts:

1. Main clause
   Min lærer vil forklare sammenligningen
   = My teacher will explain the comparison

2. Subordinate time clause
   når timen begynder
   = when the lesson begins

So the full structure is:

• [Main clause], [subordinate clause]

This is a very common Danish pattern.

Yes. You could say:

Når timen begynder, vil min lærer forklare sammenligningen.

That means the same thing.

Notice what happens in the main clause after the subordinate clause comes first:

• Når timen begynder, vil min lærer forklare sammenligningen.

Here, vil comes before min lærer because Danish main clauses follow the verb-second rule. The first position is taken by the subordinate clause, so the verb comes next.

That word order shift is very important in Danish.

At forklare is the standard Danish verb for to explain.

Forms:

• at forklare = to explain
• forklarer = explains / am explaining / is explaining
• forklarede = explained
• har forklaret = has explained

In your sentence:

• vil forklare = will explain

So it is simply the infinitive after the modal verb vil.