This is the 515th most frequent Icelandic word.
reglum
"Reglum" is the dative plural form of "regla," meaning "rules" in English.
Here, 'reglum' means 'rules' in the context of adhering to set guidelines, used in dative plural.
Við fylgjum alltaf reglum í skólanum.
We always follow the rules at school.
In this context, 'reglur' is used in the nominative plural, indicating the subject of the sentence.
Ragna kynnti nýjar reglur fyrir hópnum.
Ragna presented new rules to the group.
In this sentence, 'reglustiku' (related to 'regla') signifies a measuring tool, differing slightly as a related noun, in dative singular.