Learn Icelandic: Blómin og trén eru úti. - The flowers and the trees are outside.

You might be thinking of feminine singular nouns, which often take -in to mean "the" (like bókin). However, blóm is a neuter noun! For most neuter nouns, the plural word is the exact same as the singular word (one blóm, two blóm). To make a plural neuter noun definite ("the flowers"), the rule is to add -in. So, blóm + in = blómin.

How does tré (tree) become trén ("the trees")?

Just like blóm, tré is a neuter noun where the singular and plural forms are identical (one tré, two tré). Usually, we add -in to make plural neuter nouns definite. But because tré already ends in a vowel, adding -in would sound awkward (tréin). Instead, we drop the "i" and just add -n to get trén.

I learned that út means "outside". Why is it úti here?

Icelandic has different words for location (where something is resting) and motion (where something is moving towards). Úti is used when something is already located outside, just like the flowers and trees in this sentence. You would use út if there was movement involved, like walking out of a house.

Why do we use eru instead of er?