Стоит только повернуть баррель, как дротик меняет направление.

Here стоит только + infinitive is an idiomatic pattern meaning one only has to..., all it takes is..., or as soon as....

So:

• Стоит только повернуть баррель... = You only have to turn the barrel...
• or more naturally in English: As soon as you turn the barrel...

This is not the literal meaning of стоить as to cost.

In this sentence, стоит is being used impersonally. Russian often uses 3rd person singular neuter-style forms in set expressions without naming a subject.

So стоит только + infinitive functions like a fixed grammatical pattern:

• Стоит только начать... = One only has to start...
• Стоит только нажать кнопку... = You just need to press the button...

There is no real grammatical subject here, so стоит stays in the default 3rd person singular form.

Because the construction стоит только + infinitive requires an infinitive.

So the pattern is:

• стоит только сделать X, как происходит Y

Here:

• повернуть = to turn

The infinitive expresses the action in a general way, without specifying who does it. English often uses you or one in translation, but Russian does not need to.

Повернуть is perfective, while поворачивать is imperfective.

Here the perfective makes sense because the sentence refers to a single completed triggering action:

• повернуть = to turn once / to turn so that the action is completed

In this pattern, Russian often prefers the perfective because the meaning is:

• once that action is done,
• the next thing immediately happens.

If you used поворачивать, it would suggest an ongoing or repeated action, which does not fit as well here.

It is in the accusative case, because повернуть takes a direct object: turn what? → баррель.

But for many inanimate masculine nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: баррель
• accusative: баррель

That is why the form does not visibly change.

No. Here как is a conjunction, not the question word how.

In the pattern стоит только..., как..., it means something like:

• and then
• when / as soon as
• ...and immediately...

So:

• Стоит только повернуть баррель, как дротик меняет направление.
• As soon as you turn the barrel, the dart changes direction.

This is a set paired structure in Russian.

Because the sentence contains two clauses:

1. Стоит только повернуть баррель
2. как дротик меняет направление

Russian separates these clauses with a comma. Also, the pair стоит только..., как... is a standard correlative construction, and the comma is required.

Because the sentence describes a general rule, regular effect, or immediate consequence.

So меняет here means:

• changes
• will change
• changes right away

Russian often uses the present tense for things that are generally true or happen whenever a condition is met.

English might also say:

• the dart changes direction
• the dart will change direction

Both can fit, but Russian naturally uses меняет here.

Yes, but the meaning and style would be slightly different.

• меняет направление = changes direction
• изменяет направление = also changes direction, but often sounds a bit more formal or technical

In everyday usage, менять направление is very common and natural.

Because дротик is the subject of the second clause:

• дротик меняет направление = the dart changes direction

The subject of a finite verb in Russian normally appears in the nominative case, so дротик stays nominative.

Yes. Russian leaves it unstated because it is generic.

English usually needs something like:

• if you turn the barrel
• if one turns the barrel

But Russian can simply say повернуть баррель in this construction, with the doer understood from context.

So the meaning is not missing anything; it is just more compact than English.

It can be, but it depends on the context.

Баррель is a borrowed word and is most commonly known in Russian as barrel in the sense of a unit like a barrel of oil. In some technical, gaming, or translated contexts, it may also be used for a physical barrel component.

Depending on the object being described, other Russian words might be more common, such as:

• ствол for the barrel of a gun
• бочка for a large barrel/cask

So баррель is understandable, but whether it sounds fully natural depends on the wider context.