Barnet är rädd för mörka rum.

Barn means child in the basic, indefinite form.

• ett barn = a child
• barnet = the child

The ending -et is the definite ending for many ett-words. So in this sentence, barnet specifically means the child.

In Swedish, nouns belong to one of two grammatical genders:

• en words
• ett words

Barn is an ett word:

• ett barn
• barnet

This is something you usually have to learn with the noun. There is not always a simple rule you can rely on.

Är means is / are / am, depending on context. It is the present tense of vara, which means to be.

Examples:

• Jag är trött. = I am tired.
• Barnet är rädd. = The child is afraid.
• De är här. = They are here.

Rädd means afraid. Here it is a predicate adjective, because it comes after är.

In Swedish, many adjectives change form depending on the noun, but rädd is a little irregular. Its common forms are:

• en rädd man
• ett rädd barn
• rädda barn

So in Barnet är rädd, rädd is the correct form.

Because Swedish uses the expression rädd för, which means afraid of.

So:

• rädd för hundar = afraid of dogs
• rädd för mörkret = afraid of the dark

This is something you should learn as a set phrase:

• vara rädd för = to be afraid of

A native English speaker may want to translate word-for-word, but the important thing is that Swedish uses för here.

Because rum is plural here.

The adjective mörk changes like this:

• ett mörkt rum = a dark room
• mörka rum = dark rooms

In plural, Swedish adjectives usually take -a, so:

• mörka = dark before a plural noun

Because some Swedish nouns have the same form in singular and plural.

For rum:

• ett rum = a room
• flera rum = several rooms

So the noun itself does not change, but other words around it help show number:

• ett mörkt rum = singular
• mörka rum = plural

The adjective tells you clearly that rum is plural here.

Not by itself in this sentence. Mörka rum here means dark rooms in an indefinite plural sense.

Compare:

• mörka rum = dark rooms
• de mörka rummen = the dark rooms

To make it definite in Swedish, you usually need both:

• a definite marker like de
• adjective ending -a
• a definite noun form like rummen

The dictionary form is mörk, meaning dark.

You might see these forms:

• mörk for an en word in singular
• mörkt for an ett word in singular
• mörka for plural and for many definite forms

Examples:

• en mörk skog = a dark forest
• ett mörkt rum = a dark room
• mörka rum = dark rooms

The word order is the normal Swedish statement pattern:

• Barnet = subject
• är = verb
• rädd för mörka rum = description/complement

So it follows the same basic order as English:

• The child is afraid of dark rooms.

Swedish often uses verb-second word order, but in a simple sentence like this, that just gives you:

• subject + verb + rest

That would be:

Ett barn är rädd för ett mörkt rum.

Notice the changes:

• barnet becomes ett barn = a child
• mörka rum becomes ett mörkt rum = a dark room

This shows both the indefinite form and the singular ett adjective ending -t in mörkt.

A rough guide:

• rädd sounds a bit like redd, but with the Swedish ä vowel, which is more open than English e in many accents.

A few pronunciation notes for the whole sentence:

• Barnet: the r and a are not exactly like English
• är: sounds somewhat like air in some dialects, but not exactly
• rädd: short vowel + double consonant, so the vowel is short
• mörka: the ö is a Swedish vowel with no exact English equivalent
• rum: pronounced with a long u sound that is also not exactly like English

If you are learning pronunciation, it helps to listen to native audio, because vowels like ä and ö are very important in Swedish.

Yes. That means The child is afraid of the dark.

Compare:

• rädd för mörka rum = afraid of dark rooms
• rädd för mörkret = afraid of the darkness / the dark

So the original sentence focuses on dark rooms, while mörkret refers more generally to darkness.