Формовка закончилась быстро.

Because формовка is a feminine singular noun.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: закончился
• feminine: закончилась
• neuter: закончилось
• plural: закончились

So:

• формовка закончилась
• урок закончился
• занятие закончилось
• переговоры закончились

The ending -ась / -лась here shows feminine singular past.

It is in the nominative case, because it is the subject of the sentence — the thing that ended.

So in Формовка закончилась быстро, the structure is basically:

• Формовка = the subject
• закончилась = ended / came to an end
• быстро = quickly

Yes, закончилась is a reflexive verb form from закончиться.

In Russian, many events are expressed with a reflexive verb where English would simply say ended:

• Урок закончился. = The lesson ended.
• Встреча закончилась. = The meeting ended.
• Формовка закончилась. = The molding/forming process ended.

This does not mean the subject literally did something to itself in a very concrete way. In many cases, reflexive verbs in Russian just function as normal dictionary verbs with their own meaning.

Compare:

• закончить что-то = to finish something
  
  • Он закончил работу. = He finished the work.
• закончиться = to end, to be over
  
  • Работа закончилась. = The work ended.

The difference is mainly transitive vs. intransitive:

• закончить = to finish something
• закончиться = to end / to be over

Examples:

• Они закончили проект. = They finished the project.
• Проект закончился. = The project ended.

In your sentence, формовка is not the object of someone’s action. It is the event itself, so Russian uses закончилась.

Быстро is an adverb, meaning quickly.

It describes how the action happened:

• закончилась быстро = ended quickly

It does not change its form here. Russian adverbs like быстро, медленно, внезапно, хорошо are usually invariable.

Compare:

• быстрый = fast, quick (adjective, masculine)
• быстрая = fast, quick (adjective, feminine)
• быстро = quickly / fast (adverb)

So:

• быстрая формовка = a quick molding process
• формовка закончилась быстро = the molding process ended quickly

Yes, both are possible.

Russian word order is more flexible than English. Both of these are natural:

• Формовка закончилась быстро
• Формовка быстро закончилась

The difference is mostly one of focus or style, not basic grammar.

• Формовка закончилась быстро puts быстро in a slightly more natural end position, where it sounds like a final comment on the whole event.
• Формовка быстро закончилась can sound a bit more directly focused on the speed of ending.

In neutral written Russian, the version with быстро at the end is very common.

Закончилась is perfective.

Its dictionary form is закончиться, which describes a completed event. That fits this sentence because the process is presented as having reached its end.

Russian aspect matters a lot:

• заканчивалась = was ending / used to end
• закончилась = ended, finished completely

Compare:

• Формовка заканчивалась около пяти часов. = The molding process was ending around five o’clock.
• Формовка закончилась в пять часов. = The molding process ended at five o’clock.

Since your sentence states a finished result, perfective is the normal choice.

Yes, often you can.

• Формовка закончилась быстро
• Формовка кончилась быстро

Both can mean roughly the same thing: the process ended quickly.

However, закончиться often sounds a bit more neutral or standard, while кончиться can be slightly more conversational depending on context.

In many situations, закончиться is the safer choice for learners.

A helpful approximate pronunciation is:

for-MOF-ka zon-CHEE-las' BIST-ra

A few useful notes:

• Формовка: stress on мо́ → формо́вка
• закончилась: stress on чи → зако́нчилась
• быстро: stress on бы́ → бы́стро

If you want to sound more natural, pay attention to the stressed syllables more than trying to pronounce every unstressed vowel exactly as written.

It sounds natural, especially in a technical, industrial, or process-related context.

The noun формовка is not an everyday casual word for all situations, but in the right field it is completely normal. The whole sentence is a straightforward Russian way to report that a process ended quickly.

It has a very typical Russian pattern:

• [process/event] + закончился / закончилась / закончилось + adverb

For example:

• Проверка закончилась быстро.
• Сборка закончилась быстро.
• Загрузка закончилась быстро.

So grammatically and stylistically, the sentence is perfectly normal.