Learn Icelandic: Vanmátturinn var yfirþyrmandi. - The powerlessness was overwhelming.

The prefix van- indicates a lack, deficiency, or the exact opposite of a quality, much like "un-", "under-", or "mal-" in English. Here, it attaches to máttur (power or strength) to create vanmáttur, meaning "powerlessness" or "inability."

What is the root word máttur related to?

It is the direct cognate of the English word "might" (as in strength). When combined with the prefix, vanmáttur essentially means a lack of might.

What kind of word is yfirþyrmandi and how is it built?

It is a present participle acting as an adjective. It comes from the verb yfirþyrma (to overwhelm), which is formed from yfir (over) and þyrma (to cover or storm). The -andi suffix is the Icelandic equivalent of the English "-ing" ending, making it translate perfectly to "overwhelming."

If the subject were plural or feminine, would yfirþyrmandi change its ending to match?

No, it would stay exactly the same. Because yfirþyrmandi is a present participle ending in -andi, it is completely indeclinable. This means it never changes its form, regardless of the gender, number, or case of the noun it describes.