Глину начали центровать.

Because глину is the accusative singular form of глина.

Here, глина / clay is the direct object of the verb центровать, so Russian uses the accusative case.

• nominative: глина
• accusative: глину

This is a very common pattern for feminine nouns ending in -а:

• книга → книгу
• машина → машину
• глина → глину

Начали is the past tense plural of начать, meaning to begin or to start.

So:

• начали центровать = began to center / started centering

A useful thing to notice is that Russian past tense shows number and, in the singular, gender:

• начал = he began
• начала = she began
• начало = it began
• начали = they began

The subject is not stated.

Russian often leaves out the subject when it is already known from context, not important, or just indefinite. So this sentence can mean something like:

• They began centering the clay
• The potters began centering the clay
• People started centering the clay

The verb form начали tells you the missing subject is plural.

Because after начать Russian normally uses an infinitive to say to begin doing something.

So the structure is:

• начать + infinitive
• начали центровать = began to center

This works just like many other Russian verbs:

• начали работать = began to work
• начали говорить = began to speak
• начали делать = began to do/make

After начать, Russian normally uses an imperfective infinitive, because beginning refers to the start of a process.

So:

• начали центровать sounds natural
• a perfective infinitive here would usually sound wrong or much less natural

Compare:

• начали делать = correct
• начали сделать = not normal

The idea is: you begin a process, and imperfective verbs are the usual way to express that process.

It is a fairly specialized verb. In this context it refers to centering the clay, especially in pottery: placing and stabilizing the clay so it spins properly on the wheel.

It is related to центр meaning center.

A learner may also see related verbs in other contexts, such as центрировать or отцентрировать, but центровать is very natural in technical or craft language like pottery.

Yes, Начали центровать глину is also possible.

Russian word order is much more flexible than English word order. Putting глину first often makes the clay the topic of the sentence, especially if the previous context was already about the clay.

So the difference is usually about focus and information structure, not basic meaning.

Very roughly:

• Глину начали центровать. = As for the clay, they began centering it.
• Начали центровать глину. = They began centering the clay.

No. Grammatically, it is active.

The sentence does not mean that the clay itself performed the action. It means that someone began doing something to the clay.

So Russian is literally closer to:

• They began centering the clay

Even if English might sometimes prefer a passive-style translation in context, the Russian grammar here is still active with an omitted subject.

The normal stress is:

Гли́ну нача́ли центрова́ть.

A rough pronunciation guide:

GLEE-noo na-CHA-li tsen-tra-VAHT'

For learners, the most important stresses to remember are:

• гли́ну
• нача́ли
• центрова́ть