Learn Icelandic: Við klöppum fyrir þessum leikurum.

The preposition fyrir (for) forces the following words into the dative case here. The dative plural form of the demonstrative pronoun þessi (this/these) is þessum. A great bonus for learners: þessum is the dative plural form for all three genders (masculine, feminine, and neuter)!

I thought þessum meant "this" in the singular. Can it also mean "these"?

Yes! The word þessum does double duty. It is the dative singular form for masculine and neuter nouns, but it is also the dative plural form for all genders. Since leikurum (actors) is plural, þessum translates to "these" in this sentence.

Does the preposition fyrir always trigger the dative case?

No, fyrir can take either the accusative or the dative case depending on the context. When it implies doing an action for someone's benefit or to show appreciation (like clapping for them), or being physically "in front of" something, it takes the dative. That is why both the pronoun and the noun here take dative endings.

What is the dictionary form of leikurum, and why does it end in -um?