Læreren forklarer ordklassen, når vi er i tvivl.

Læreren means the teacher. The ending -en is the definite article added to the noun.

• en lærer = a teacher
• læreren = the teacher

In Danish, the definite article is usually attached to the end of the noun rather than written as a separate word like English the.

Yes. Forklarer is the present tense of at forklare = to explain.

• at forklare = to explain
• forklarer = explains / is explaining

In Danish, the present tense often ends in -r.

So Læreren forklarer ordklassen means The teacher explains the word class or The teacher is explaining the word class, depending on context.

Ordklassen means the word class or the part of speech.

It comes from:

• ord = word
• klasse = class
• ordklasse = word class / part of speech
• ordklassen = the word class / the part of speech

Depending on context, this could refer to a category like noun, verb, adjective, and so on.

Because Danish usually puts definiteness at the end of the noun.

• en ordklasse = a word class
• ordklassen = the word class

So English uses a separate word, but Danish often uses a suffix instead.

Yes, når usually means when in sentences like this.

Here it introduces a time-related subordinate clause:

• når vi er i tvivl = when we are in doubt / when we are unsure

In many everyday contexts, English speakers will naturally translate i tvivl as unsure rather than the more literal in doubt.

Because after når, Danish uses normal subordinate clause word order.

This clause is:

• vi = we
• er = are
• i tvivl = in doubt / unsure

So the structure is straightforward: subject + verb + complement.

A useful thing to know is that in Danish subordinate clauses, sentence adverbs such as ikke often come before the verb:

• når vi ikke er i tvivl = when we are not in doubt

That is different from main clause word order, where the verb usually comes earlier.

Literally, i tvivl means in doubt, but in natural English it often means:

• unsure
• uncertain
• not sure

So:

• vi er i tvivl = we are unsure / we are in doubt / we are uncertain

This is a very common Danish expression, and it is worth learning as a set phrase.

Because Danish uses the fixed expression at være i tvivl = to be in doubt / to be unsure.

So Danish says:

• jeg er i tvivl = I am unsure
• hun er i tvivl = she is unsure
• vi er i tvivl = we are unsure

This is idiomatic. English learners should treat være i tvivl as a chunk.

Because Danish spelling rules normally use a comma before a subordinate clause.

Here:

• main clause: Læreren forklarer ordklassen
• subordinate clause: når vi er i tvivl

So the comma helps mark the boundary between the two clauses.

English comma use is less regular in similar sentences, so this often stands out to learners.

Yes. You can say:

Når vi er i tvivl, forklarer læreren ordklassen.

That means the same basic thing: When we are unsure, the teacher explains the word class.

But notice the word order in the main clause after the fronted subordinate clause:

• Når vi er i tvivl, forklarer læreren ...

The verb forklarer comes before the subject læreren. This is because Danish main clauses follow the verb-second pattern.

Not always in every nuance, but very often, yes.

In many learner situations, it is enough to think:

• når = when

However, Danish can distinguish between når and da in some contexts:

• når is often used for repeated events, general conditions, or future situations
• da is often used for a single event in the past

In this sentence, når fits because it means something like whenever we are unsure or when that situation happens.

Yes, that is a very natural interpretation.

Læreren forklarer ordklassen, når vi er i tvivl can suggest:

• The teacher explains the word class when we are unsure
• The teacher explains the word class whenever we are unsure

English may choose whenever if the meaning is habitual or repeated.

Because er i tvivl describes a state: are unsure.

• er = are / is
• bliver = become / get

So:

• vi er i tvivl = we are unsure
• vi bliver i tvivl = we become unsure

The original sentence is talking about the state of being unsure, not the change into that state.

A few natural translations could be:

• The teacher explains the word class when we are unsure.
• The teacher explains the part of speech when we are unsure.
• When we are unsure, the teacher explains the word class.
• The teacher explains it when we are in doubt.

If you want the most natural everyday English, unsure is often smoother than the more literal in doubt.

A rough guide for English speakers is something like LAIR-uh-rən, but that is only approximate.

A few points:

• æ in lærer is a front vowel that does not match standard English exactly
• the r sounds in Danish are different from English r
• final syllables are often reduced in natural speech

So the best approach is to listen to native audio and repeat it, rather than relying too much on English-style spelling.