Det blå mærke under øjet er mindre i dag.

Because det blå mærke means the blue mark / the bruise.

In Danish, when a noun is definite and has an adjective before it, you usually need:

• a definite article like den/det/de
• the adjective in its definite form
• and often the noun itself in its definite form in other contexts, though after den/det/de + adjective, Danish normally keeps the noun in its base form

So:

• et blåt mærke = a bruise / a blue mark
• det blå mærke = the bruise / the blue mark

Here mærke is a neuter noun, so the definite article is det.

Because the adjective changes depending on the grammar.

With an indefinite neuter singular noun, you would say:

• et blåt mærke

But in the sentence you have a definite noun phrase:

• det blå mærke

In definite noun phrases, adjectives usually take -e in Danish, but some adjectives have irregular-looking forms. Here, blå is the form used before the noun in the definite phrase.

So compare:

• et blåt mærke = a bruise
• det blå mærke = the bruise

Mærke literally means mark.

But et blåt mærke is the normal Danish expression for a bruise. Literally, it is something like a blue mark.

So although the words are transparent, you should learn blåt mærke as a common expression meaning bruise.

Because øjet is the definite form of øje.

• et øje = an eye
• øjet = the eye

So:

• under øjet = under the eye

This is very normal in Danish. Body parts are often expressed with the definite form where English might also say the.

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

With øje, the definite ending gives you:

• øje = eye
• øjet = the eye

So under øjet already means under the eye. You do not need a separate article there.

This is one of the big differences from English.

Mindre is the comparative form meaning smaller / less.

It comes from lille (small / little), and this adjective is somewhat irregular:

• lille = small
• mindre = smaller
• mindst = smallest

So:

• er mindre i dag = is smaller today

This is completely standard Danish.

Yes. Mindre can mean both less and smaller, depending on context.

In this sentence, because it refers to a bruise, English naturally translates it as smaller:

• Det blå mærke ... er mindre i dag = The bruise ... is smaller today

But in other sentences, mindre can clearly mean less.

That is the most neutral word order here.

The structure is:

• Det blå mærke under øjet = subject
• er = verb
• mindre = complement
• i dag = time expression

So the sentence is arranged very naturally as:

• The bruise under the eye is smaller today.

You could move i dag for emphasis, but the version given is straightforward and standard.

Under øjet tells you where the bruise is.

It functions as a prepositional phrase:

• under = under
• øjet = the eye

So det blå mærke under øjet means:

• the bruise under the eye

It describes which bruise we are talking about.

Yes, but it changes the meaning.

• et blåt mærke under øjet = a bruise under the eye
• det blå mærke under øjet = the bruise under the eye

So the sentence uses det because it refers to a specific bruise, not just any bruise.

Because Danish is a V2 language, which means the finite verb normally comes in second position in main clauses.

Here the first element is the whole subject:

• Det blå mærke under øjet = first element

Then comes the verb:

• er

So the order is perfectly normal:

• Det blå mærke under øjet er mindre i dag.

If another element were moved to the front, the verb would still stay second:

• I dag er det blå mærke under øjet mindre.

A careful approximation is:

• deh bloh MAIR-keh OON-er OY-et air MIN-druh ee day

A few notes:

• det is often pronounced more like deh in everyday speech
• blå has a long å sound, somewhat like blaw without a strong w
• mærke has the Danish æ, a front vowel somewhat like the a in cat, but not exactly
• øjet contains øj, which can be tricky for English speakers
• dag often sounds closer to dai than to a fully pronounced hard g

You do not need perfect pronunciation right away, but ø, æ, and soft consonants are worth paying attention to.