Learn Icelandic: Seiglan gerir afrekið þeim mun stærra. - The perseverance makes the achievement all the greater.

It translates to "all the" or "that much" when used before a comparative adjective. Literally, it means "by that difference". Þeim is the dative form of the demonstrative pronoun sá (that), and mun is the dative of munur (difference). Here, it emphasizes how much greater the perseverance makes the achievement.

Why is the comparative adjective stærra used instead of stærri?

The adjective describes afrekið (the achievement), which is a neuter noun. In the comparative degree, Icelandic adjectives only have two endings: -a for neuter words and -i for masculine and feminine words. Therefore, stærri becomes stærra to match the neuter gender of afrekið.

Is afrekið in the nominative or accusative case here?

It is in the accusative case because it is the direct object of the verb gerir (makes). The form afrekið looks identical in both nominative and accusative because neuter nouns do not change their spelling between these two cases. The adjective stærra is also in the accusative to agree with it.

Could I use því instead of þeim here, since we say því stærra, því betra (the bigger, the better)?