Learn Icelandic: Við köllum þetta bindandi reglu.

The verb kalla (to call) is one of a few Icelandic verbs that take a double accusative. This means it requires the accusative case for both the object you are naming (þetta) and the name or description you are giving it (bindandi reglu).

Since regla is a feminine word, shouldn't þetta be the feminine þessa?

If you were saying "We call this rule binding," you would indeed use the feminine form to agree with the noun: Við köllum þessa reglu bindandi. But in this sentence, þetta is acting as a standalone, generic pronoun meaning "this concept" or "this thing." In Icelandic, undefined abstract concepts or unknown objects always default to the neuter gender.

Why doesn't the adjective bindandi change to match the feminine, accusative noun reglu?

Words ending in -andi are present participles (equivalent to English words ending in "-ing", like "binding"). When used as adjectives, they are completely indeclinable in Icelandic. This means bindandi will never change its ending, regardless of the gender, number, or case of the noun it modifies.

Why is there an ö in köllum when the dictionary form of the verb is kalla?