Спасательный плот и спасательная шлюпка проверены.

Because the adjective has to agree with the noun it describes.

• плот is masculine, so the adjective is спасательный
• шлюпка is feminine, so the adjective is спасательная

This is normal Russian adjective agreement. In the nominative singular:

• masculine: -ый / -ий / -ой
• feminine: -ая / -яя
• neuter: -ое / -ее

So the adjective changes form depending on the gender of the noun.

They are in the nominative case.

That is because they are the subject of the sentence: the things being talked about are the life raft and the lifeboat.

You can tell from the basic dictionary forms:

• плот
• шлюпка

And their adjectives also match the nominative singular forms:

• спасательный
• спасательная

Because together they make a compound subject: the life raft and the lifeboat.

Even though each noun is singular by itself, the whole subject is plural because it refers to two things. So the predicate is also plural:

• плот проверен = the raft has been checked
• шлюпка проверена = the boat has been checked
• плот и шлюпка проверены = the raft and the boat have been checked

This is similar to English: The raft and the boat are..., not is.

Проверены is the short-form passive participle, plural, from проверить / проверенный.

In practical terms, it means something like:

• have been checked
• are checked
• sometimes were checked, depending on context

Short-form passive participles are very common in Russian, especially in formal, technical, and written styles.

Compare:

• проверенный = full form, like checked / verified
• проверены = short predicate form, used as the main predicate of the sentence

So in this sentence, проверены is the main predicative word: have been checked / are checked.

Because in Russian, the verb to be is usually omitted in the present tense.

So where English says:

• The life raft and lifeboat are checked
• The life raft and lifeboat have been checked

Russian can simply say:

• Спасательный плот и спасательная шлюпка проверены.

There is no separate present-tense are.

If you wanted to make the past more explicit, you could say:

• Спасательный плот и спасательная шлюпка были проверены.

That means more clearly were checked / had been checked.

It depends on context.

In many real situations, especially technical or procedural language, проверены often implies a completed check with a present result, so English often translates it as:

• have been checked

It can also be understood as:

• are checked if the focus is on the current state
• were checked in the right context, especially if time is already understood

So the Russian form is a little broader than a single English tense. Context tells you which English version sounds best.

Because the two nouns have different genders:

• плот = masculine
• шлюпка = feminine

A single singular adjective form cannot agree with both at once. So Russian normally repeats the adjective:

• спасательный плот и спасательная шлюпка

That is the most natural way here.

If both nouns had the same gender and structure, sharing might be easier in some contexts, but with mixed genders, repeating the adjective is the normal and clearest choice.

Yes, you could.

Russian word order is more flexible than English. Both of these are possible:

• Спасательный плот и спасательная шлюпка проверены.
• Проверены спасательный плот и спасательная шлюпка.

The first version is more neutral if you are simply stating information.

The second can sound a bit more formal, report-like, or focused on the result checked.

So the sentence order can change, but the original order is very normal.

Because Russian has no articles.

English distinguishes between:

• a life raft
• the life raft

Russian does not have separate words for that. Whether something is definite or indefinite is understood from context.

So:

• спасательный плот can mean a life raft or the life raft
• спасательная шлюпка can mean a lifeboat or the lifeboat

In this sentence, context usually makes them sound definite in English: the life raft and the lifeboat.

The participle would change to match the number, and in the singular it also matches gender.

For example:

• Спасательный плот проверен. = The life raft has been checked.
• Спасательная шлюпка проверена. = The lifeboat has been checked.

Notice:

• masculine singular: проверен
• feminine singular: проверена
• plural: проверены

So the plural проверены is used because the sentence has two items together.