Learn Dutch: Alles staat al stil door de staking. - Everything is already standing still because of the strike.

The verb here is stilstaan (to stand still), which is a separable verb. In a main clause, the conjugated part (staat) goes in the second position, and the separable prefix (stil) gets pushed toward the end of the sentence. Time particles like al (already) typically come before the separated prefix.

Alles means everything, which sounds plural. Why does it use the singular verb staat?

Just like in English, the Dutch word alles (everything) is grammatically singular. Therefore, it takes the singular verb conjugation staat, rather than the plural form staan.

The translation says 'because of the strike'. Why use door instead of omdat?

Omdat is a conjunction that must introduce a completely new clause with a subject and a verb at the end. Door is a preposition used directly before a noun phrase (like de staking) to mean 'because of' or 'due to' an external cause.

Could I use nog instead of al in this sentence? How would that change the meaning?

Yes, you could say Alles staat nog stil door de staking. However, it changes the meaning from 'already' to 'still'. Using al implies the standstill happened sooner than expected, whereas nog means the standstill is ongoing and simply hasn't ended yet.