Стоит ему достать конструктор, как он забывает про телефон и книги.

Yes, by itself стоит can mean costs or is standing, but not in this sentence.

Here it is part of the fixed pattern:

Стоит + dative + infinitive, как ...

This means:

• as soon as ...
• all it takes is for ... and ...
• the moment ...

So:

Стоит ему достать конструктор, как он забывает ...
= As soon as he takes out the construction set, he forgets ...

It is an idiomatic structure, so it should be learned as a whole.

Because this construction requires the person to be in the dative case.

So in:

Стоит ему достать конструктор ...

ему means for him / to him, but in natural English we usually just translate it as he:

• Стоит мне начать ... = As soon as I start ...
• Стоит тебе увидеть это ... = As soon as you see this ...
• Стоит ему достать конструктор ... = As soon as he takes out the construction set ...

So ему is not the subject in the nominative; it is the dative noun/pronoun required by the pattern.

No, those are different.

• Стоит ему достать ..., как ... = As soon as he takes out ..., ... happens
• Ему стоит достать ... = He should take out ... or It would be worth his taking out ...

So word order matters a lot here.

In your sentence, стоит is part of the idiomatic стоит ..., как ... structure. It is not giving advice.

Because достать is perfective, and that fits this pattern very well.

Here the idea is: one completed action immediately triggers another.

• достать = to take out / get out as a single completed act
• доставать = to be taking out / to take out repeatedly / habitually

In Стоит ему достать конструктор, как ..., the meaning is basically:

the moment he takes it out

So the perfective infinitive достать is the natural choice.

No. Here как does not mean how.

It is the second part of the correlative structure:

Стоит ..., как ...

Together, this means something like:

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

So in this sentence, как introduces the main clause that happens immediately after the first action.

A rough literal sense is:

It only takes him to take out the construction set, and he forgets ...

Because the sentence describes a habitual / repeated situation.

забывает is imperfective present, and here it means something like:

• he tends to forget
• he forgets every time
• whenever this happens, he forgets

So the whole sentence describes a regular pattern in his behavior.

If you wanted a single future event, you could use забудет:

• Стоит ему достать конструктор, как он забудет про телефон и книги.
  = As soon as he takes out the construction set, he’ll forget about his phone and books.

But in your sentence, забывает makes it sound general and habitual.

Yes, both are possible, but they are slightly different in style.

With забывать / забыть, Russian often uses:

• забывать про + accusative
• забывать о + prepositional

So:

• забывает про телефон и книги
• забывает о телефоне и книгах

Both mean forgets about the phone and books.

The version with про is very common and conversational.
The version with о can sound a bit more neutral or bookish.

Because про takes the accusative case.

So:

• про телефон
• про книги

These forms happen to look the same as the dictionary forms because:

• телефон is masculine inanimate singular, and its accusative = nominative
• книги is inanimate plural, and its accusative = nominative

The same thing happens with конструктор after достать:

• достать конструктор

That is also accusative, but it looks the same as the basic form because конструктор is masculine inanimate singular.

In this sentence, конструктор means a construction set / building set, usually a toy for children, like Lego or a similar set of pieces for building things.

It does not usually mean constructor in the English sense of a person who constructs something.

So here it refers to the toy/object he takes out and gets absorbed in.

Because Russian often uses the present tense to describe a general repeated pattern.

This sentence is not about one specific moment only. It means something like:

• Whenever he takes out the construction set, he forgets about his phone and books
• As soon as he gets the construction set out, he always forgets about the phone and books

So the present tense here expresses a regular characteristic or repeated behavior, not just what is happening right now.

To some extent, yes, but the core pattern should stay recognizable:

Стоит ему достать конструктор, как он забывает про телефон и книги.

This is the most natural and clear version.

Russian word order is fairly flexible, but with this construction, learners should keep the standard pattern:

• Стоит + dative + infinitive, как + clause

That will sound the most natural and help avoid confusion with other meanings of стоить.