Спасательная шлюпка уже проверена.

Because it is the subject of the sentence: the lifeboat is the thing being described.

• спасательная шлюпка = lifeboat
• Nominative singular is the normal form for the subject

So the sentence is built around what has the property of being already checked: the lifeboat.

Because it agrees with шлюпка, which is feminine singular.

проверена is a short-form passive participle, and it changes for gender and number:

• masculine: проверен
• feminine: проверена
• neuter: проверено
• plural: проверены

Since шлюпка is feminine, Russian uses проверена.

It is the short passive participle of проверить.

In this sentence, it means something like:

• checked
• inspected
• verified
• tested

The exact English word depends on context. For a lifeboat, checked or inspected is usually the most natural translation.

This form describes the resulting state after the action has been completed.

Russian normally leaves out the verb to be in the present tense.

So where English says:

• The lifeboat is already checked
• The lifeboat has already been checked

Russian can simply say:

• Спасательная шлюпка уже проверена.

Literally, it is closer to Lifeboat already checked, but natural English requires is or has been.

Grammatically, it is a present-time predicative statement: the lifeboat is now in the state of having been checked.

That is why English often translates it with the present perfect passive:

• has already been checked

But depending on context, is already checked can also work.

So it is not a simple past like was checked. The sentence focuses on the current result.

Because проверена usually implies a completed action with a present result.

The sentence is not just describing a general property of the lifeboat. It means that someone has completed the checking, and now that result matters.

So:

• is checked can sound a bit static or generic in English
• has already been checked better captures the idea of completion

That is why the present perfect passive is often the best translation.

Because the underlying verb here is проверить, the perfective verb.

Perfective aspect focuses on:

• completion
• a finished result
• a single completed check

That matches this sentence perfectly: the lifeboat is already checked.

If you wanted to talk about the process instead, Russian would use a different structure, for example:

• Спасательную шлюпку проверяют. = They are checking the lifeboat.
• Спасательную шлюпку уже проверяли. = They have checked / were checking the lifeboat before (depending on context)

So проверена points to a completed result, not an ongoing process.

This is a very common and important question.

• проверена = short form, usually used as the predicate
  
  • Шлюпка проверена. = The lifeboat has been checked.
• проверенная = full form, usually used before a noun like an adjective
  
  • проверенная шлюпка = a checked / tested lifeboat

So:

• short form = the lifeboat is checked
• full form = a checked lifeboat

In other words, проверена tells you something about the lifeboat, while проверенная directly modifies the noun.

уже means already.

In Спасательная шлюпка уже проверена, it naturally modifies the whole predicate: the checking has already been completed.

This word order is neutral and natural.

Russian word order is flexible, but different placements can change emphasis:

• Спасательная шлюпка уже проверена. = neutral
• Спасательная шлюпка проверена уже. = possible, but more marked; it may sound contrastive or stylistic
• Уже спасательная шлюпка проверена. = unusual in most contexts

So the given order is the most standard one.

Because Russian has no articles.

Russian does not have direct equivalents of a and the. Whether the noun is definite or indefinite is understood from context.

So спасательная шлюпка can mean:

• a lifeboat
• the lifeboat

In this sentence, English will usually say the lifeboat, because the context normally makes it a specific one.

Yes. You can add the agent, usually in the instrumental case.

For example:

• Спасательная шлюпка уже проверена инспектором. = The lifeboat has already been checked by the inspector.
• Спасательная шлюпка уже проверена экипажем. = The lifeboat has already been checked by the crew.

In many passive sentences, the doer is omitted because it is unknown, obvious, or unimportant.

Yes, very often.

A very common alternative is:

• Спасательную шлюпку уже проверили.

Literally, that means They already checked the lifeboat, but in natural English it is often just The lifeboat has already been checked.

This active plural form is common when the speaker does not care to name who did it.

So:

• Спасательная шлюпка уже проверена. = more passive, result-focused, slightly more formal or report-like
• Спасательную шлюпку уже проверили. = very natural conversational alternative

Both are correct; they just present the information a little differently.