Learn Icelandic: Barnið les á gólfinu. - The child reads on the floor.

The preposition á (meaning on) forces the noun that follows it into the dative case when describing a fixed location. Gólfið is the standard (nominative) form for the floor, but the dative form is gólfinu.

How do we get the ending -inu for gólfinu?

Gólf (floor) is a neuter noun. In this lesson, we learn that to make a singular neuter noun definite (meaning the floor) in the dative case, you add the ending -inu to the base word.

Why do barnið and gólfinu have different definite endings if they are both neuter nouns?

It is because of their different jobs in the sentence. Barnið is the subject (the one doing the reading), so it stays in the default nominative case, which adds -ið. Gólfinu follows the preposition á, which forces it into the dative case, so it takes the dative neuter ending -inu.

What is the dictionary form of the verb les?

The dictionary form (infinitive) is lesa (to read). It becomes les here because the subject, barnið, is a third-person singular noun (equivalent to he, she, or it).