"Lille" er et tillægsord, men i flertal ændrer nogle tillægsord form.

Because the sentence is talking about the word lille, not using it to describe a noun.

Compare:

• en lille bil = here lille is actually being used as an adjective
• Lille er et tillægsord = here lille is being mentioned as a vocabulary item

English does the same thing:

• Small is an adjective.

So lille is being treated as a word under discussion.

Tillægsord is the Danish grammatical term for an adjective.

An adjective describes a noun, for example:

• en lille hund = a small dog
• et stort hus = a big house

So in this sentence, et tillægsord means an adjective.

Because tillægsord is a neuter noun in Danish, so it takes et.

In Danish, nouns are either:

• common gender: take en
• neuter gender: take et

So:

• en bil
• et hus
• et tillægsord

This is something you usually just have to learn with the noun.

I flertal means in the plural.

It is a fixed expression:

• ental = singular
• flertal = plural

So:

• i ental = in the singular
• i flertal = in the plural

It is describing the grammatical number, not the meaning of “many” in a general sense.

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

After men, the clause is still a main clause. If i flertal is placed first, then the verb must come next:

• I flertal ændrer nogle tillægsord form.

Structure:

• i flertal = first element
• ændrer = finite verb in second position
• nogle tillægsord = subject

An English speaker often expects the subject before the verb, but Danish often moves another element to the front and keeps the verb second.

You could also say:

• Men nogle tillægsord ændrer form i flertal.

That is also grammatical, but the emphasis is slightly different.

The common plural form is små.

Examples:

• en lille bil = a small car
• et lille hus = a small house
• små biler = small cars
• små huse = small houses

So lille is irregular, because you do not get a plural like lillee or something similar. Instead, it changes to a different form: små.

Many Danish adjectives do, but not always in the same way.

A very common pattern is:

• common singular: base form
  
  • en stor bil
• neuter singular: -t
  
  • et stort hus
• plural / definite: -e
  
  • store biler
  • den store bil

So yes, plural agreement is very normal in Danish adjectives.

However, the sentence says some adjectives because:

• some adjectives are irregular
• some adjectives may be indeclinable in certain uses
• the speaker may be focusing especially on words like lille, which do not follow the most basic pattern

So the important idea is: Danish adjectives often agree with the noun, and plural can affect their form.

In Danish, it is very common to leave out a possessive word like deres when the meaning is already clear.

So:

• ændrer form = change form
• ændrer deres form = change their form

Both are understandable, but ændrer form sounds natural and concise in a general grammar explanation.

English often prefers change form or change their form depending on context, and Danish often uses the shorter version.

No. Nogle means some, not all.

So:

• nogle tillægsord = some adjectives
• alle tillægsord = all adjectives

This matters because the sentence is not making a universal claim about every adjective in every situation. It is saying that there are adjectives for which the plural form is different.