Skólinn er rólegur.

Icelandic doesn’t use a standalone definite article. Instead, it adds a suffix to the noun.
• skóli = “school” (indefinite)
• skólinn = skóli + -inn (definite masc. nom. sg.) = “the school”

Adjectives in predicate position take the strong declension and must agree in gender, number, and case with the subject.
• skólinn is masculine singular nominative, so the matching strong form is rólegur.
• A neuter noun would use rólegt, a feminine would use róleg.

No—those endings don’t match skólinn’s gender:
• rólegt is neuter
• róleg is feminine
Always choose the adjective form that agrees: rólegur for masc. sing.

Approximate English pronunciation:
• Skólinn = “SKOH-lin” (long “ó” like English “oh”)
• er = “ehr” (short, like the “ir” in “sir”)
• rólegur = “ROH-leh-gur” (stress on the first syllable)
IPA: /ˈskouː.lɪn ˈɛr ˈrou.lɛ.ɣʏr/

Put ekki right after the finite verb:
• Skólinn er ekki rólegur. = “The school is not calm.”
If you add objects or adverbs, ekki still follows the conjugated verb.

Move the verb er to the front (Icelandic V2 rule):
• Er skólinn rólegur? = “Is the school calm?”

• rólegur = “calm,” “peaceful,” describing atmosphere or mood
• hljóðlátur = “quiet,” “making little noise,” more about sound level
Example:
• Skólinn er rólegur → the school is calm (low stress, easygoing)
• Skólinn er hljóðlátur → the school is quiet (not noisy)

Use the plural forms of both verb and adjective:
• Skólar eru rólegir. = “(Some) schools are calm.”
• Skólarnir eru rólegir. = “The schools are calm.”
Here eru is 3rd pl. of vera and rólegir is masc. pl. nominative strong.

That’s an attributive adjective, which follows different (weak) inflections and placement:
• rólegi skólinn = “the calm school” (weak form, masc. sg.)
Alternatively, a more formal style:
• hinn rólegi skóli = “the calm school” (using the demonstrative hinn + strong form)