Learn Icelandic: Ef áreynslan er mikil verður þú þeim mun einbeittari. - If the effort is great, you will become all the more focused.

The phrase þeim mun translates literally to 'by that difference' but is used here to mean 'all the more' or 'that much'. Þeim is the dative form of það (that), and mun is the dative form of the noun munur (difference). It acts as an intensifier for the comparative adjective einbeittari (more focused), showing that the focus increases in proportion to the effort.

Is the word mun in this sentence related to the future tense helper verb?

No, they are just homonyms. The future helper verb comes from the verb munu (as in ég mun fara). The mun in this phrase is actually the dative singular form of the masculine noun munur, which means 'a difference' or 'an amount'.

Why is the word order verður þú instead of þú verður?

This is the classic V2 (verb-second) rule in action. The entire subordinate clause Ef áreynslan er mikil acts as the first block of information in the sentence. To keep the main verb in the second position, verður must immediately follow that clause, which pushes the subject þú directly after the verb.

Could I use ert instead of verður here?