Learn Hardcore Latin: Omnes mirantur quam cito nebula super flumen veniat. - Everyone wonders how quickly the mist comes over the river.

Grammatically, omnes is nominative plural, so it is the subject of mirantur. Latin often uses a plural adjective by itself to mean all people when the noun is understood.

So:

omnes mirantur = everyone is amazed / all people wonder

It is plural in form, even if English often translates it with the singular idea everyone.

Why is the verb mirantur plural?

Because its subject, omnes, is plural.

omnes = all people
mirantur = they wonder / they are amazed

So the agreement is straightforward:

plural subject → plural verb

Even if you translate omnes as everyone, Latin is still thinking in terms of all persons, so the verb is plural.

Why does mirantur look passive if the meaning is active?

Because miror, mirari is a deponent verb.

A deponent verb:

has passive forms
but an