Стоит поставить лишний пробел, как форма показывает ошибку.

Here стоит is part of the fixed pattern стоит + infinitive, как...

In this pattern, it means something like:

• it only takes...
• as soon as...
• once...

So Стоит поставить лишний пробел, как форма показывает ошибку means:

• As soon as you enter an extra space, the form shows an error.
• literally: It only takes putting an extra space for the form to show an error.

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

This is a common Russian construction showing that one action immediately causes another.

Structure:

Стоит + infinitive, как + result clause

Meaning:

• as soon as...
• once...
• the moment...

So in your sentence:

• Стоит поставить лишний пробел = As soon as you put an extra space
• как форма показывает ошибку = the form shows an error

It often implies a very quick or automatic result.

Compare:

• Стоит мне закрыть окно, как становится душно.
  As soon as I close the window, it gets stuffy.

• Стоит ему начать говорить, как все замолкают.
  As soon as he starts talking, everyone falls silent.

Поставить is perfective, and that fits the meaning here because the sentence refers to a single completed action that triggers a result.

• ставить = imperfective, more like to be putting / to put repeatedly
• поставить = perfective, to put once / to place

Here the idea is:

• one extra space gets entered
• then the form immediately reacts

So the completed action is what matters.

If you used ставить, it would sound less natural in this sentence because the construction is about a specific event, not an ongoing process.

Because лишний пробел is the direct object of поставить.

• поставить что? → лишний пробел

So it is in the accusative case.

But пробел is an inanimate masculine noun, and for inanimate masculine nouns, the accusative singular looks the same as the nominative singular:

• nominative: лишний пробел
• accusative: лишний пробел

So the case is accusative, even though the form looks unchanged.

Yes, it is natural, especially in typing, editing, and computer-related contexts.

Literally, поставить пробел means to put/add a space.

Russian often uses verbs like поставить, вставить, or добавить in this kind of context:

• поставить запятую = put a comma
• поставить точку = put a period
• поставить пробел = insert/add a space

So лишний пробел is an extra space, and поставить лишний пробел means to accidentally add an extra space.

Yes, как often means how, but here it is a conjunction in the fixed construction стоит..., как...

In this pattern, как means something closer to:

• and then
• when
• as soon as

So you should not translate it as how here.

This is one of those cases where the whole pattern matters more than the individual word.

Because the sentence has two clauses:

1. Стоит поставить лишний пробел
2. как форма показывает ошибку

The comma separates these two parts of the construction.

Russian punctuation normally places a comma before conjunctions introducing a new clause, and this idiomatic pattern also requires it.

There is no explicit subject, and that is normal here.

This is an impersonal way of speaking. The meaning is general:

• as soon as someone puts an extra space
• as soon as you put an extra space
• if you put an extra space

Russian often leaves the doer unspecified when the meaning is general or obvious from context.

So the sentence is not about one particular person; it means any user / anyone.

It means that the form displays some kind of validation error or error message.

• форма = form, usually an online form or input form
• показывает ошибку = shows an error

In natural English, depending on context, you might translate it as:

• the form shows an error
• the form displays an error
• the form flags it as an error
• the form gives an error

Russian often says показывать ошибку where English might prefer give an error or show an error message.

It is fairly neutral and natural, especially in written explanations or technical descriptions.

The pattern Стоит + infinitive, как... can sound a little more literary or polished than very plain everyday speech, but it is still common and understandable.

More conversational alternatives might be:

• Как только поставишь лишний пробел, форма показывает ошибку.
• Если поставить лишний пробел, форма сразу показывает ошибку.
• Поставишь лишний пробел — форма показывает ошибку.

So your sentence is perfectly good Russian, just a bit more structured/stylistic than the simplest spoken version.

Sometimes yes, with a small change in style.

For example:

• Стоит поставить лишний пробел, и форма показывает ошибку.

This also means:

• As soon as you put an extra space, the form shows an error.

The version with как often emphasizes the immediate result a bit more strongly and belongs to a very common fixed pattern. The version with и is also natural and may feel slightly more straightforward.

It can often be translated both ways, depending on style and context.

Possible translations:

• As soon as you enter an extra space, the form shows an error.
• If you enter an extra space, the form shows an error.
• The moment you add an extra space, the form shows an error.

The Russian construction usually suggests an immediate, automatic consequence, so as soon as or the moment often captures the nuance better than plain if. But in many practical contexts, if is still a perfectly acceptable translation.