Learn Icelandic: Margir komu þjótandi undir borðið. - Many came rushing under the table.

This structure pairs a basic motion verb like koma (to come) or fara (to go) with the present participle of another verb. Here, komu (came) provides the main direction, while the present participle þjótandi (from þjóta, meaning to rush or dash) acts like an adverb to tell us exactly how that movement happened.

Does the present participle þjótandi change endings to match the plural subject margir?

No, it stays exactly the same. Present participles ending in -andi are invariable when used in this manner of motion construction. Whether one person or many people are rushing, it remains þjótandi.

Why is the table (borðið) in the accusative case here instead of the dative?

The preposition undir dictates the accusative case when there is active movement toward a destination. Because the subjects are moving from outside to underneath the table, it takes the accusative. If they were already under the table and just rushing around in that confined space, undir would take the dative case (undir borðinu).

Why does margir stand alone without a noun?

In Icelandic, adjectives like margur can easily act as standalone pronouns meaning 'many people.' It takes the masculine nominative plural form (margir) here because masculine plural is the default grammatical gender used when referring to a mixed or unspecified group of people.