Sólin er björt en skýin eru þung.

In Icelandic, the definite article is very often attached to the end of the noun instead of written as a separate word.

• sól = sun
• sólin = the sun

And similarly:

• ský = cloud / clouds, depending on the form
• skýin = the clouds

So where English says the sun, Icelandic often says a single word, sólin.

The ending that means the changes depending on gender, number, and case.

Because skýin is plural, while skýið would be singular.

For the noun ský:

• skýið = the cloud
• skýin = the clouds

In your sentence, the verb is eru, which is plural, so the subject must also be plural: skýin.

A useful comparison:

• Skýið er þungt = the cloud is heavy
• Skýin eru þung = the clouds are heavy

That contrast helps show both the noun and the adjective changing for singular vs. plural.

These are two present-tense forms of the verb vera, meaning to be.

• er = is
• eru = are

So:

• Sólin er björt = the sun is bright
• Skýin eru þung = the clouds are heavy

English does the same thing:

• the sun is
• the clouds are

So this part is actually quite familiar for an English speaker.

In Icelandic, every noun has a grammatical gender, and you usually have to learn it together with the noun.

• sól is feminine
• ský is neuter

This matters because gender affects:

• the form of the definite article
• the form of adjectives
• sometimes pronouns and other related words

A dictionary will normally show the gender for you, often as:

• f. for feminine
• n. for neuter
• m. for masculine

So it is best to learn nouns as complete items, for example:

• sól, f.
• ský, n.

Because Icelandic adjectives must agree with the noun they describe in gender, number, and case.

Here the dictionary forms are:

• bjartur = bright
• þungur = heavy

But in the sentence they change:

• sólin is feminine singular, so bjartur becomes björt
• skýin is neuter plural, so þungur becomes þung

So:

• Sólin er björt
• Skýin eru þung

A very common learner question is why it is þung and not þungt.
The answer is that þungt is neuter singular, while þung here is neuter plural.

Compare:

• skýið er þungt = the cloud is heavy
• skýin eru þung = the clouds are heavy

They are in the nominative case.

That is because:

• sólin and skýin are the subjects of the sentence
• after vera in a sentence like this, the predicate adjectives also stay nominative

So we have nominative forms all through the sentence:

• sólin
• björt
• skýin
• þung

This is one reason the sentence is a nice beginner example: it uses straightforward subject + to be + adjective structure.

En means but.

It joins two main clauses:

• Sólin er björt
• skýin eru þung

So the whole sentence is basically:

• clause 1 + en
  • clause 2

In this kind of simple sentence, en does not force any unusual word order. Each clause keeps normal Icelandic order:

• subject + verb + complement

So the structure is very close to English:

• The sun is bright, but the clouds are heavy

A few parts are especially worth noticing:

• ó in Sólin: roughly like o in go, but long
• ö in björt: no exact English equivalent; similar to German ö
• ý in skýin: roughly like ee
• þ in þung: like th in thin

A rough English-style approximation of the whole sentence is:

• SOH-lin er byurt en SKEE-in EH-ru thung

But that is only approximate.

Two extra pronunciation tips:

• Icelandic words are usually stressed on the first syllable
• accented vowels such as ó and ý are not just decoration; they matter for both pronunciation and spelling

So o and ó, or i and í, are different letters, not just stylistic variants.