In de soep zitten nog een wortel en twee champignons.

Yes, zitten literally means to sit, but in Dutch it is also often used more broadly for being located somewhere, especially when talking about things in a container, mixture, space, or enclosed area.

So in In de soep zitten nog een wortel en twee champignons, zitten does not mean the carrot and mushrooms are literally sitting like people. It means something like:

• there are still
• there remain
• there are still present

Dutch often uses verbs like zitten, staan, and liggen where English would often just use to be.

Because Dutch very often prefers a position/location verb instead of plain zijn when describing where something is.

Here, zitten works well because the carrot and mushrooms are inside the soup. Dutch speakers often use zitten for things that are:

• in something
• stuck in something
• contained in something
• mixed into something

So In de soep zijn nog een wortel en twee champignons would sound unnatural.
Dutch strongly prefers zitten here.

Because the subject is plural overall:

• een wortel
• en
• twee champignons

Together, that makes a compound subject, so Dutch uses the plural verb:

• een wortel en twee champignons zitten

Even though the first noun is singular (een wortel), the whole subject is more than one thing, so the verb is plural.

Because Dutch main clauses follow the V2 rule: the finite verb must come in the second position.

Here the speaker puts In de soep first for emphasis or as the topic:

• In de soep = first position
• zitten = second position
• nog een wortel en twee champignons = the rest

A more neutral version is:

• Er zitten nog een wortel en twee champignons in de soep.

Both are correct. The given sentence just starts with the location.

Here nog means still or yet.

It suggests that the carrot and the two mushrooms are still in the soup, often with the idea that they:

• have not been removed
• remain there
• are still present

So the sentence has the sense of:

• There are still a carrot and two mushrooms in the soup.

Without nog, it would simply state that they are in the soup, without that extra idea of still / remaining.

Because een is the indefinite article a/an, and you only use it with a singular countable noun:

• een wortel = a carrot

With a number like twee, you do not add een:

• twee champignons = two mushrooms

So:

• een wortel
• twee champignons

is exactly what you would expect.

Because soep is a de-word in Dutch:

• de soep

This is something learners usually just have to memorize with the noun. There is no reliable one-rule-fits-all translation from English for whether a Dutch noun takes de or het.

So the correct phrase is:

• in de soep

Yes, absolutely. That is a very natural sentence.

Compare:

• Er zitten nog een wortel en twee champignons in de soep.
• In de soep zitten nog een wortel en twee champignons.

The version with er is often a bit more neutral and common when introducing that something exists somewhere.
The version starting with In de soep puts more focus on the location.

In English, both might be translated similarly.

In that kind of sentence, er is an expletive or dummy subject, similar to English there in:

• There are two mushrooms in the soup.

So:

• Er zitten twee champignons in de soep. = There are two mushrooms in the soup.

Dutch often uses er when presenting the existence of something somewhere.

Because in Dutch, short adverbs like nog often come before the noun phrase they modify or before the new information in the sentence.

So:

• zitten nog een wortel en twee champignons

sounds natural.

It marks that what follows is still present.
Putting nog somewhere else may change the emphasis or sound less natural.

Yes, champignon is the normal Dutch word for the common cultivated mushroom, and the plural is:

• champignons

It is a borrowing from French, which is why it may look a little unusual to English speakers.

If you are talking specifically about the usual white or brown cooking mushrooms, champignon/champignons is exactly the right everyday word.

The structure is:

• In de soep = prepositional phrase / location
• zitten = finite verb
• nog = adverb
• een wortel en twee champignons = subject

So a rough structural gloss is:

• In the soup
  • are
    • still
      • a carrot and two mushrooms

This looks unusual to an English speaker because English usually wants the subject earlier, but in Dutch this is completely normal because of the verb-second pattern.

Normally, no. Zitten is the best choice here.

Dutch has several common location verbs:

• staan = stand
• liggen = lie
• zitten = sit / be in / be contained in

For ingredients in soup, zitten is the idiomatic choice because they are in the soup, as part of it or contained by it.

So:

• In de soep zitten ... = natural
• In de soep liggen ... = usually not the normal choice here
• In de soep staan ... = not appropriate here

Yes, it can. Because of nog, the sentence can easily suggest that these ingredients are still left in the soup.

For example, someone might say it when noticing what remains:

• In de soep zitten nog een wortel en twee champignons.

That could mean:

• those ingredients are still there
• they have not been eaten yet
• they remain in the bowl/pot

The exact nuance depends on context, but nog often gives that still remaining feeling.