In English, we often follow a preposition directly with an action ending in "-ing" (e.g., "talk about updating" or "start by turning on"). In Icelandic, prepositions cannot attach directly to a verb. They need a noun or pronoun to connect to.
To bridge this gap, Icelandic inserts the pronoun það (it) directly after the preposition, followed by að and the infinitive verb. Because prepositions control grammatical case, það will change its form (decline) depending on which preposition you use.
Accusative: um það að
When a verb phrase uses a preposition that governs the accusative case, like um (about), the pronoun remains það.
Dative: því að
Many prepositions in these types of expressions govern the dative case. This includes á (on/by), yfir (over/about), að (at/to), eftir (after/for), and fyrir (for/of). After these prepositions, the pronoun það changes to its dative form, því.
Genitive: þess að
Prepositions like til (to/for) and án (without) govern the genitive case. After these, the pronoun becomes þess.
The phrase til þess að is incredibly common and functions as the direct equivalent of the English phrase "in order to."