Если светофор не работает, нужно действовать осторожно.

Because светофор is the thing doing the action grammatically, so Russian uses the normal 3rd person singular form of the verb:

• светофор работает = the traffic light works / is working
• светофор не работает = the traffic light does not work / is not working

The form работается is a different, less common structure with a different meaning, often used in expressions like Мне хорошо работается = I work well / It’s easy for me to work. It would not be used for a machine or device simply functioning or not functioning.

In Russian, the present tense often covers both:

• does not work
• is not working

So не работает can mean either:

• the traffic light is generally out of order
• the traffic light is currently not functioning

Russian does not require a separate continuous form like English is working. Context tells you which meaning is intended.

Если means if.

It introduces a condition:

• Если светофор не работает, ... = If the traffic light is not working, ...

It is the standard word for real or possible conditions in Russian. Very common patterns are:

• Если будет время, я приду. = If there is time, I’ll come.
• Если он позвонит, скажи мне. = If he calls, tell me.

Russian normally uses a comma between the if-clause and the main clause:

• Если светофор не работает, нужно действовать осторожно.

This is standard punctuation in Russian complex sentences. English also often uses a comma when the if-clause comes first:

• If the traffic light isn’t working, you need to act carefully.

So the comma here is required in normal writing.

Нужно means something like:

• it is necessary
• one must
• you need to

Russian often uses impersonal constructions, where there is no explicit subject like English it or you.

So:

• нужно действовать осторожно literally = it is necessary to act cautiously
• natural English = you need to act carefully / one must act carefully

This structure is very common in Russian:

• Нужно подождать. = We need to wait / It is necessary to wait.
• Нужно быть внимательным. = You need to be attentive.

Yes. Надо and нужно are often very close in meaning.

So these are both natural:

• Если светофор не работает, нужно действовать осторожно.
• Если светофор не работает, надо действовать осторожно.

In many everyday situations, they are interchangeable. Very roughly:

• надо can sound a bit more conversational
• нужно can sound a bit more neutral or formal

But the difference is small here.

Because after нужно, Russian usually uses the infinitive.

Pattern:

• нужно + infinitive

So:

• нужно действовать = need to act
• нужно ждать = need to wait
• нужно смотреть = need to watch

A form like действуете means you act / you are acting and would require a stated subject or a different sentence structure.

An imperative like действуй means act!, which would be a command, not the same structure as it is necessary to act.

Действовать means to act, to proceed, or to behave in practice. In safety instructions, it often means take action in an appropriate way.

So действовать осторожно is a very natural expression in formal or instructional language:

• act cautiously
• proceed carefully

You could sometimes also see verbs like:

• быть осторожным = to be careful
• вести себя осторожно = to behave carefully

But действовать осторожно sounds especially natural in rules, instructions, and official wording.

Because осторожно is an adverb, and it describes how the action is done:

• действовать осторожно = to act cautiously / carefully

Compare:

• осторожный = careful, cautious as an adjective for a noun
  
  • осторожный водитель = a careful driver
• осторожным = an adjective form used in certain case patterns
  
  • будь осторожным = be careful
• осторожно = adverb
  
  • ехать осторожно = to drive carefully

Since it modifies the verb действовать, the adverb осторожно is the correct form.

Not always exactly, but here it is very close.

Осторожно often means:

• carefully
• cautiously
• with caution

It often suggests avoiding danger or risk. In this traffic-related sentence, cautiously may be especially close in tone.

The given word order is the most neutral and natural:

• Если светофор не работает, нужно действовать осторожно.

Russian word order is fairly flexible, but changing it may affect emphasis. For example:

• Нужно действовать осторожно, если светофор не работает.

This is also correct. It places the main point first: you need to act cautiously.

So both are grammatical, but the original version is very natural when giving a condition first.

Because светофор is the grammatical subject of работает:

• светофор работает = the traffic light works

Subjects are normally in the nominative case in Russian.

You can recognize the basic structure:

• кто? что? = who? what?
• что работает? = светофор

So nominative is expected here.

Only if the context already makes it completely clear what is not working.

In isolation, Если не работает, нужно действовать осторожно would sound incomplete, because the listener would ask: What isn’t working?

Russian often omits words that are understood from context, but in a standalone instruction like this, светофор should be included.

It is mostly neutral to slightly formal.

That is because of phrases like:

• нужно действовать
• осторожно

This sounds very natural in:

• public safety instructions
• driving rules
• official notices
• educational materials

In very casual speech, a person might say something simpler, such as:

• Если светофор не работает, будь осторожен. = If the traffic light isn’t working, be careful.

But the original sentence is perfectly natural and standard.

Yes, very often. It is one of the most common ways to say that something is not functioning.

Examples:

• Телефон не работает. = The phone isn’t working.
• Лифт не работает. = The elevator isn’t working.
• Компьютер не работает. = The computer isn’t working.

So светофор не работает follows a very common Russian pattern.

A fairly literal breakdown would be:

• Если = if
• светофор = traffic light
• не работает = does not work / is not working
• нужно = it is necessary / need to
• действовать = to act
• осторожно = cautiously / carefully

So a literal version would be:

• If the traffic light is not working, it is necessary to act cautiously.

That is more literal than natural English, but it shows how the Russian sentence is built.