Этот стояк уже проверили.

Because стояк is a masculine singular noun, so the demonstrative must also be masculine singular.

• masculine: этот
• feminine: эта
• neuter: это

So:

• этот стояк = this riser / this pipe / this standpipe
• not эта стояк, because эта is feminine
• not это стояк, because это is neuter

Russian demonstratives have to agree with the noun in gender, number, and case.

In this sentence, стояк is the direct object of проверили, so it is in the accusative case.

But стояк is:

• masculine
• singular
• inanimate

For masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: стояк
• accusative: стояк

That is why the form stays the same.

The same thing happens with этот:

• nominative: этот
• accusative masculine inanimate: этот

So Этот стояк can look the same whether it is the subject or the direct object. You figure out its role from the verb and the overall sentence structure.

Because проверили does not agree with стояк here.

The verb is in the past plural form, and the subject is not stated. Russian often uses 3rd person plural like this when the people who did the action are unknown, unimportant, or obvious from context.

So Этот стояк уже проверили literally means something like:

• They already checked this riser

But in natural English, it is often better translated as:

• This riser has already been checked

So the plural verb does not mean there were several risers. It means some people checked it.

There is no explicit subject.

Russian often leaves out the subject when it is:

• unknown
• unimportant
• general
• obvious from context

Here, the sentence uses an indefinite-personal construction:

• проверили = they checked
• but they is not actually said

This is very common in Russian. Similar examples:

• Меня позвали. = They called me.
• Документы уже отправили. = They already sent the documents.
• Его не пустили. = They didn’t let him in.

In English, these are often translated with passive voice.

This is a question of aspect.

• проверили = perfective
• проверяли = imperfective

Here, проверили is used because the sentence focuses on the completed result: the checking has already been done.

So:

• Этот стояк уже проверили. = This riser has already been checked.
• the action is complete

If you said проверяли, it would usually suggest:

• the process of checking
• repeated checking
• incomplete or background action
• less focus on the finished result

For example:

• Этот стояк уже проверяли, но проблему не нашли.
  = They already checked this riser before, but they didn’t find the problem.

So with уже, проверили is the natural choice when you mean it’s done already.

Уже means already.

It shows that the action happened before now, often sooner than expected or earlier than a certain point in the conversation.

So:

• Этот стояк уже проверили. = This riser has already been checked.

It can also carry a slight sense of:

• this has been dealt with
• no need to do it again
• that part is finished

Its exact nuance depends on context.

Yes, the word order can change.

Russian word order is more flexible than English, because case endings do a lot of the grammatical work. The basic meaning usually stays the same, but the focus or emphasis changes.

Here:

• Этот стояк уже проверили.
  Neutral: This riser has already been checked.

Possible alternatives:

• Уже проверили этот стояк.
  More emphasis on already checked
• Этот стояк проверили уже.
  Can sound more marked, often conversational, with emphasis on already
• Проверили уже этот стояк.
  Also possible in speech, depending on what is being contrasted

The version you were given is very natural and neutral.

Grammatically, it is active.

The verb проверили is an active past-tense verb form meaning they checked. But because the subject is omitted and indefinite, the sentence often feels passive-like in English.

So Russian uses an active form:

• Этот стояк уже проверили.

But English often prefers a passive translation:

• This riser has already been checked.

Russian could also use a more explicitly passive-style expression in some contexts, for example:

• Этот стояк уже проверен.
  = This riser has already been checked.

That version puts more emphasis on the state/result. The original sentence sounds a bit more like someone already did that.

Both can be translated as This riser has already been checked, but they are not identical.

Этот стояк уже проверили

• literally: They already checked this riser
• emphasizes the action
• implies that some people performed the check

Этот стояк уже проверен

• emphasizes the resulting state
• means the riser is now in a checked condition
• often sounds more formal or concise

Very roughly:

• проверили = focus on what people did
• проверен = focus on the current result

Both are correct in the right context.

Not in this sentence.

Even though этот стояк looks like a nominative form, the verb tells you what is happening. Since проверили is a plural past-tense verb with an omitted subject, этот стояк is understood as the object: the thing that was checked.

If этот стояк were the subject, the verb would need to agree with it:

• Этот стояк уже проверен. = This riser is already checked
• or another singular predicate structure

But Этот стояк уже проверили means someone checked this riser, so этот стояк is the object.

You can add the people who did the action.

For example:

• Этот стояк уже проверили сантехники.
  = The plumbers already checked this riser.

• Этот стояк уже проверили специалисты.
  = The specialists already checked this riser.

• Этот стояк уже проверила комиссия.
  = The commission already checked this riser.

Notice that if you name the subject, the verb must agree with it:

• сантехники = plural → проверили
• комиссия = feminine singular → проверила

Yes, very common.

Russian often uses:

• a plural past verb
• with no stated subject

to mean that someone did something, without naming who. This is especially common when the doer is unknown, irrelevant, or obvious.

So sentences like these are very natural:

• Дверь уже закрыли. = The door has already been closed.
• Счёт отправили. = The bill has been sent.
• Ваш заказ уже подготовили. = Your order has already been prepared.

This is one of the most useful patterns to get used to in everyday Russian.