Learn Icelandic: Málefnið gagnast engum núna. - The cause benefits no one now.

The verb gagnast (to benefit) dictates the case of the object that follows it, requiring the dative case. Because of this, we must use engum, which is the dative form of the indefinite pronoun enginn.

Is engum a singular or plural word in this sentence?

When referring to 'no one' or 'nobody' in a general, unspecified way, Icelandic defaults to the masculine singular. So here, engum is the masculine dative singular. (Note: engum also happens to be the dative plural for all genders, but grammatically it acts as a singular here).

Why does the verb gagnast end with -st?

It is a middle voice verb, which is why it has that -st ending. The active verb gagna (to do good) exists, but the -st form gagnast is very commonly used to mean 'to prove useful to' or 'to benefit.' Like many -st verbs, it takes a dative object.

What is the base noun for málefnið (the cause) and what is its gender?

The base word is málefni, which is a neuter noun. Because it ends in a vowel, we just add -ð to make it definite (the cause). If it were masculine or feminine, the definite ending would look different, such as -inn or -in.