Learn Icelandic: Þú finnur safnið á þessu torgi. - You find the museum on this square.

The preposition á (on) triggers the dative case when you are talking about a fixed location. Because the noun torg (square) is a neuter word, the demonstrative pronoun changes to the neuter dative form, which is þessu.

Why does torg have an -i at the end?

The -i is a very common ending for neuter nouns in the dative case. Since the preposition á requires the dative here, torg becomes torgi.

Does the preposition á always take the dative case?

Not always! When á means being in or on a fixed location (like being located on a square), it takes the dative. If it describes motion or direction onto something, it takes the accusative. Here, the museum is simply located on the square, so we use the dative.

Why isn't there a 'the' attached to torgi, like there is in safnið?

In Icelandic, when you use a demonstrative word like þessu (this), the noun that follows usually drops the definite article. That is why it is just á þessu torgi and not á þessu torgið. Meanwhile, safnið stands alone as 'the museum', so it keeps its attached article -ið.