Когда светофор станет зелёным, можно спокойно перейти улицу.

Because after the verb стать in the sense of to become / to turn, Russian normally uses the instrumental case for the resulting state.

So:

• стать зелёным = to become green / to turn green

Here:

• зелёный = green
• masculine singular instrumental = зелёным

This is very common in Russian:

• стать врачом = to become a doctor
• стать ясным = to become clear
• стать старше = to become older

So светофор станет зелёным literally means the traffic light will become green.

Станет comes from стать, which means to become / to turn into. In this sentence, the important idea is the change from one color to another.

• Когда светофор станет зелёным = When the traffic light turns green
• Когда светофор будет зелёным = When the traffic light is green / will be green

Both can make sense, but станет is more natural when talking about the moment the light changes. It focuses on the transition, not just the state.

Перейти is the perfective verb, so it presents the crossing as a single completed action: crossing from one side to the other.

• перейти улицу = cross the street completely

This fits the sentence well, because it means that once the light turns green, it is fine to go ahead and cross.

If you used переходить, the meaning would be less about one completed crossing and more about the process, habit, or general activity of crossing.

Compare:

• можно перейти улицу = you can cross the street
• можно переходить улицу = it is permissible to be crossing / to cross in general

In this context, перейти is the more natural choice.

Можно is an impersonal word meaning things like:

• it is possible
• it is allowed
• one may
• you can

There is no explicit subject in the Russian sentence. English often supplies a general you, but Russian does not need that here.

So:

• можно спокойно перейти улицу = you can safely cross the street / it is safe to cross the street

The exact English translation depends on context, but grammatically Russian is simply using an impersonal construction.

Because улицу is the direct object of перейти.

The dictionary form is:

• улица = street

Its feminine singular accusative form is:

• улицу

So:

• перейти улицу = to cross the street

This is standard for a transitive verb taking a direct object.

Literally, спокойно means calmly. But in this kind of context, it often means something like:

• safely
• without worry
• with peace of mind

So можно спокойно перейти улицу is not just about emotional calmness. It suggests that once the light is green, crossing is safe and unproblematic.

Because Когда светофор станет зелёным is a subordinate clause introduced by когда.

Russian normally separates subordinate clauses with commas:

• Когда светофор станет зелёным, можно спокойно перейти улицу.

This is similar to English writing:

• When the traffic light turns green, you can safely cross the street.

So the comma is required here.

Когда means when, and если means if.

Russian uses когда because the speaker treats the light turning green as an expected event, not an uncertain condition.

• Когда светофор станет зелёным = when the light turns green
• Если светофор станет зелёным = if the light turns green

Since traffic lights are expected to change normally, когда is the natural choice.

Yes. Russian word order is fairly flexible, though not completely free. The sentence as given is natural and neutral:

• Когда светофор станет зелёным, можно спокойно перейти улицу.

It puts the time condition first, which is very common.

You could also say:

• Можно спокойно перейти улицу, когда светофор станет зелёным.

This is still grammatical, but the emphasis is slightly different. The original version sounds more natural if you want to set up the condition first: once the light turns green...

Russian often leaves out a general subject when English would use you, one, or people.

Here the idea is general:

• when the traffic light turns green, one can / you can cross the street safely

Russian expresses that with the impersonal можно and an infinitive:

• можно перейти

So the sentence does not need ты, вы, or any other subject pronoun.

Yes, literally that is exactly what it means.

Word by word:

• Когда = when
• светофор = traffic light
• станет = will become / turns
• зелёным = green, in the instrumental
• можно = it is possible / allowed / you can
• спокойно = calmly / safely
• перейти = to cross
• улицу = street

Natural English usually prefers turn green rather than become green, but the literal structure is very transparent here.

The correct spelling is зелёным, with ё. In many Russian texts, especially informal ones, ё is often written as е, so you may also see:

• зеленым

But the pronunciation and meaning are the same here. For a learner, keeping ё is helpful because it shows the stress and the correct sound more clearly:

• зелёным

So if you see зеленым in real texts, it is usually not a different word—just a simplified spelling convention.