В гончарной мастерской стоит гончарный круг.

Because after в when it means in/inside a place, Russian normally uses the prepositional case.

So:

• гончарная мастерская = a pottery workshop / potter’s studio (dictionary form, nominative)
• в гончарной мастерской = in the pottery workshop (prepositional)

Both words change because the adjective has to agree with the noun:

• гончарная → гончарной
• мастерская → мастерской

This is normal adjective + noun agreement in case, gender, and number.

It is the prepositional case.

You can tell because:

• the phrase follows в in a location meaning
• it answers the question где? = where?

So:

• Где стоит гончарный круг?
• В гончарной мастерской.

Russian usually does not use есть the way English uses there is in ordinary present-tense location sentences.

So instead of saying something like In the workshop there is a wheel, Russian very naturally says:

• В мастерской стоит круг.

Here стоит literally means stands, but in many contexts it simply means an object is standing / located there.

This is extremely common with inanimate objects in Russian.

Yes, literally it does, but the meaning is often broader than in English.

Russian commonly uses different verbs depending on the object’s usual position:

• стоит = stands
• лежит = lies
• висит = hangs

With larger objects, furniture, machines, containers, and many other things, стоит is often the natural choice even when English would just say is.

So in this sentence, стоит can feel like:

• A potter’s wheel stands in the workshop
• or more naturally in English: There is a potter’s wheel in the workshop

Yes, круг often means circle, but it also has some extended meanings.

In the fixed phrase гончарный круг, it means potter’s wheel.

So this is a vocabulary item worth learning as a whole expression:

• гончарный круг = potter’s wheel

This is one of those cases where the literal meaning of a word does not tell you the full meaning of the phrase.

Because the adjective changes to match the noun it describes.

1. гончарной мастерской

   • мастерская is feminine
   • the whole phrase is in the prepositional case
   • so the adjective becomes гончарной

2. гончарный круг

   • круг is masculine
   • it is the subject, so it is in the nominative case
   • so the adjective is гончарный

This is basic adjective agreement in Russian.

Yes, you absolutely could.

Both are grammatical:

• В гончарной мастерской стоит гончарный круг.
• Гончарный круг стоит в гончарной мастерской.

The difference is mainly in focus.

• В гончарной мастерской стоит гончарный круг puts the location first and has more of a there is ... in ... feeling.
• Гончарный круг стоит в гончарной мастерской starts with the object itself, so it feels more like you are talking specifically about the wheel and saying where it is.

Russian word order is flexible, but it often changes emphasis rather than core meaning.

Because Russian has no articles.

So гончарный круг can mean:

• a potter’s wheel
• the potter’s wheel

The exact meaning depends on context.

In this sentence, English will often translate it as a potter’s wheel if the wheel is being introduced for the first time.

Мастерская means workshop, studio, or workroom, depending on context.

So гончарная мастерская can be translated as:

• pottery workshop
• potter’s workshop
• pottery studio

All of these are reasonable English translations, depending on the setting.

Not always.

With в, the case depends on the meaning:

• в + prepositional = location, in
• в + accusative = direction, into

Compare:

• В гончарной мастерской стоит круг.
  The wheel is in the workshop.
  location → prepositional

• Он вошёл в гончарную мастерскую.
  He went into the pottery workshop.
  motion into → accusative

This is a very important pattern in Russian.

It is a perfectly normal grammatical Russian sentence.

The only reason it may feel slightly textbook-like is the repetition of гончар-:

• гончарной мастерской
• гончарный круг

In real speech, if the context is already clear, a speaker might simplify:

• В мастерской стоит гончарный круг.
• В гончарной мастерской стоит круг.

But the full version is completely natural and is actually very useful for learners because it clearly shows the vocabulary and grammar.