Det, hun siger, lyder vigtigt.

In Danish, det can be used to mean that / the thing / what in this kind of structure.

So:

• det = that / the thing
• hun siger = she says

Together, det, hun siger literally feels like that which she says, and in natural English we usually translate that as what she says.

So the sentence structure is:

• Det, hun siger = the subject
• lyder vigtigt = sounds important

Because Danish often leaves the relative pronoun out when it is not the subject of the relative clause.

In this sentence, the missing word would be the object of siger:

• hun siger [det] = she says [it]

So Danish can simply say:

• det, hun siger

You may also see:

• det, som hun siger

But without som is very common and natural here.

Because hun siger is a subordinate/relative clause, and Danish subordinate clauses normally keep the order:

• subject + verb

So:

• hun siger = she says

If this were a main clause, Danish often uses verb-second word order, but this part is not a main clause. It is attached to det and describes it.

They mark the inserted relative clause.

The core sentence is:

• Det lyder vigtigt. = That sounds important.

Then Danish inserts extra information:

• hun siger = she says

So it becomes:

• Det, hun siger, lyder vigtigt.

The commas show that hun siger belongs with det and separates that inserted clause from the rest of the sentence.

A learner should also know that Danish comma styles vary a little. You may sometimes see:

• Det hun siger, lyder vigtigt.

But the version with both commas is very standard in teaching materials.

Because det is the neuter form, and in this sentence it refers to an abstract thing: what she says.

Danish often uses det for:

• ideas
• statements
• situations
• unspecified things

Since what she says is treated as a neuter idea/content, det is the natural choice.

Lyder is the present tense of lyde, which literally means to sound.

So:

• Det lyder vigtigt = That sounds important

It does not mean that it definitely is important. It means that, based on what is heard or understood, it gives the impression of being important.

So there is a difference between:

• Det er vigtigt = It is important
• Det lyder vigtigt = It sounds important

Because the adjective agrees with det, which is neuter singular.

Adjective forms in Danish often change like this:

• en vigtig ting = a important thing
• et vigtigt emne = an important topic
• det er vigtigt = it is important

Here the subject is det, so the predicate adjective takes the neuter -t form:

• vigtigt

It is an adjective.

After verbs like være, blive, virke, lyde, Danish often uses a predicate adjective:

• Det er vigtigt
• Det virker vigtigt
• Det lyder vigtigt

So vigtigt describes det. It is not functioning as an adverb.

Yes. That is grammatically correct.

Compare:

• Det, hun siger, lyder vigtigt
• Det, som hun siger, lyder vigtigt

Both work. The version without som is often more natural and lighter in everyday Danish.

So if you are speaking or writing normally, det, hun siger is a very good choice.

No, not quite.

• Hun siger noget vigtigt = She is saying something important
• Det, hun siger, lyder vigtigt = What she is saying sounds important

The second sentence is more cautious. It focuses on the impression the listener gets. It does not fully state as a fact that the content is important; it says it sounds important.

The main verb is lyder.

That is useful because learners sometimes look at siger first and think it is the main verb. But siger belongs only to the relative clause hun siger.

So the structure is:

• main subject: Det, hun siger
• main verb: lyder
• predicate: vigtigt

In other words, the whole sentence is really built around:

• [What she says] sounds important.