Если есть пострадавший, нужно сразу вызвать спасателей и открыть дверь.

Not necessarily. In Russian, the singular is often used in a general, class-like sense.

So Если есть пострадавший means something like:

• If there is an injured person
• If anyone is injured
• If there is a casualty

It does not strongly emphasize the number. If you specifically wanted to say there are multiple injured people, you could say Если есть пострадавшие.

Пострадавший is originally a participle from the verb пострадать (to suffer, to be injured, to be affected), but in modern Russian it is very commonly used like a noun.

So in this sentence, пострадавший functions as a noun meaning:

• an injured person
• a casualty
• a victim

This is very common in Russian: participles can become noun-like words.

Both are possible, but Если есть пострадавший is the more natural, neutral order here.

Russian word order is flexible, and word order often affects focus:

• Если есть пострадавший = neutral: if there is an injured person
• Если пострадавший есть = more marked, with extra emphasis on there is

So the version in the sentence sounds more natural in instructions or emergency guidance.

Russian has no articles. There is no direct equivalent of English a/an or the.

Whether пострадавший means:

• an injured person
• the injured person
• injured person(s) in a general sense

depends on the context.

Here, English would usually use an or a more general phrase like if anyone is injured, but Russian simply says пострадавший.

Нужно is an impersonal predicative word meaning:

• it is necessary
• one should
• it is needed

Russian often gives instructions without naming the subject. So:

• нужно вызвать... = it is necessary to call...
• more naturally in English: you should call...

This makes the sentence sound neutral and instructional, which is very common in manuals, safety rules, and official advice.

Because after нужно, Russian normally uses the infinitive.

Pattern:

• нужно + infinitive

So:

• нужно вызвать = it is necessary to call
• нужно открыть = it is necessary to open

This is one of the most common ways to express obligation or recommendation in Russian.

Because вызвать takes a direct object in the accusative case, and спасатели (rescuers / rescue workers) is animate.

For animate masculine nouns in the plural, the accusative plural looks like the genitive plural:

• nominative: спасатели
• accusative: спасателей

So:

• вызвать спасателей = call the rescuers / rescue workers

This is a very important rule in Russian:
animate masculine nouns often change form in the accusative.

It is in the accusative, but for дверь, the accusative singular has the same form as the nominative singular.

So:

• nominative: дверь
• accusative: дверь

This happens with many inanimate feminine nouns of this type.

So in открыть дверь, the noun is absolutely in the correct case; it just happens not to look different.

They are perfective because the sentence is talking about specific completed actions:

• вызвать спасателей = call the rescuers (make the call)
• открыть дверь = open the door (complete the opening)

In emergency instructions, Russian often uses the perfective infinitive to emphasize a single necessary action with a result.

Compare:

• вызвать / открыть = do it once, complete it
• вызывать / открывать = be calling / be opening / do it repeatedly or as a process

So the perfective is the natural choice here.

Сразу means immediately / right away / at once.

Here it modifies the instruction:

• нужно сразу вызвать... = you should immediately call...

Its position is flexible, but this placement is very natural. Russian adverbs often move around more freely than in English.

For example, these are all possible, with slightly different emphasis:

• нужно сразу вызвать спасателей
• сразу нужно вызвать спасателей
• нужно вызвать спасателей сразу

The sentence’s version is neutral and natural.

Yes. In many contexts, нужно and надо are very close in meaning.

So you could also say:

• Если есть пострадавший, надо сразу вызвать спасателей и открыть дверь.

Both mean roughly you should / it is necessary to...

A rough tendency:

• надо often sounds a little more conversational
• нужно can sound a little more neutral or formal

But in many real situations, the difference is small.

Because Если есть пострадавший is a subordinate clause introduced by если (if).

Russian normally separates this kind of clause with a comma:

• Если есть пострадавший, нужно...

This is standard Russian punctuation: subordinate clauses are usually set off by commas, even where English punctuation may feel lighter.