Learn Icelandic: Börnin eru sæl í garðinum. - The children are happy in the garden.

Adjectives in Icelandic change to match the noun they describe. Because börnin (the children) is a neuter plural noun, the adjective takes its neuter plural form, which is sæl.

How do we get the form börnin from the base word for child?

The base dictionary word is barn (a neuter noun). In the plural, the vowel changes and it becomes börn (children). To say the children, we add the neuter plural definite ending -in, giving us börnin.

Why does the word for garden change to garðinum here?

The preposition í means in. When talking about a fixed location (being somewhere rather than moving towards it), í forces the following noun into the dative case. The word garður (a masculine noun) becomes garði in the dative, and adding the definite suffix makes it garðinum.

Is there a difference between using sæl and glöð here?

You will often see both used to mean happy. The word sæll usually describes a deep, peaceful contentment or well-being, while glaður (which would become glöð here to match the neuter plural noun) is often used for a more active, outward joy.