Learn Icelandic: Orðabókin er á borðinu. - The dictionary is on the table.

In Icelandic, "the" is attached to the end of the noun, but the exact ending depends on the word's gender and grammatical case. Orðabók is a feminine noun in the nominative case (the subject of the sentence), so it takes the ending -in. Borð is a neuter noun in the dative case, so it takes -inu.

Why is the table (borðinu) in the dative case here?

The preposition á (on) triggers different cases depending on the situation. When it describes a stationary location, meaning where something currently is, it requires the dative case. If you were describing movement, like putting the dictionary onto the table, you would use the accusative case instead.

Is orðabók a combination of two different words?

Yes, it is a compound word made from orð (word) and bók (book), literally translating to "word book". A helpful rule to remember is that the gender of a compound word always comes from its final part. Since bók is feminine, orðabók is also feminine.

What if I want to say the dictionary is on "a" table instead of "the" table?

You would say Orðabókin er á borði. You still need to use the dative case because of the preposition á showing location, which changes the base word borð to borði. You just drop the definite article ending -nu that makes it "the" table.