De mist was rond de middag voorbij.

Because mist is a de-word in Dutch. Noun gender in Dutch is something you usually have to learn together with the noun.

So:

• de mist = the fog

There is no special reason visible from the form of the word here; it is just the correct article for this noun.

Rond de middag means around noon or around midday.

• rond = around
• de middag = noon / midday

It gives an approximate time, not an exact one. So the sentence does not mean the fog ended at one precise moment; it means this happened roughly around noon.

Good question. Middag as a noun is normally a de-word:

• de middag
• een warme middag

But Dutch also has fixed time expressions like:

• het is middag
• het is ochtend
• het is avond

In those expressions, het is not the article for the noun in the usual sense. It is part of the fixed pattern het is ... used for times of day or weather-type statements.

So in your sentence, because middag is used as a noun after rond, you say rond de middag.

Here voorbij is used with was to mean something like over, finished, or gone.

In this sentence, was voorbij means the fog had ended / was gone by then. English often uses different wording, such as:

• The fog was gone around noon
• The fog had cleared around noon
• The fog was over around noon

So voorbij is not literally a motion word here. It describes a state: the fog had passed.

Dutch often expresses ideas like this with zijn + adjective/complement.

So:

• De mist was voorbij = the fog was over / had passed

That focuses on the resulting state.

If you wanted to focus more on the action of disappearing, Dutch could also use other verbs, for example:

• De mist verdween rond de middag = the fog disappeared around noon
• De mist trok rond de middag op = the fog lifted around noon

Your sentence is about the situation being finished, not about the action itself.

Because Dutch main clauses normally place the finite verb in the second position.

Here:

• De mist = first element
• was = finite verb, so it comes second

After that come the other parts:

• rond de middag = time expression
• voorbij = complement

So the structure is very normal Dutch main-clause word order:

• subject + finite verb + other information

Dutch often places items like voorbij toward the end of the clause, which is why this order sounds natural.

Yes, Dutch word order is somewhat flexible, and that version is possible. But De mist was rond de middag voorbij is the more natural neutral order for many speakers.

Putting rond de middag before voorbij makes the sentence flow well:

• topic: De mist
• verb: was
• time: rond de middag
• final state: voorbij

If you move things around, the meaning stays basically the same, but the emphasis can shift slightly.

It is understandable and acceptable, but many Dutch speakers might also choose a more specific expression for fog.

Very common alternatives are:

• De mist was rond de middag verdwenen = the fog had disappeared around noon
• De mist trok rond de middag op = the fog lifted around noon
• De mist was rond de middag weg = the fog was gone around noon

So voorbij works, but with mist, forms like verdwenen, weg, or optrekken can sound a bit more idiomatic.

Because om is usually used for a specific clock time:

• om twaalf uur = at twelve o’clock
• om half drie = at half past two

But de middag is a broader time period, not a precise clock time. So Dutch normally says:

• rond de middag = around noon
• tegen de middag = by / toward noon

Om de middag is not the normal way to say this.