Когда светофор станет зелёным, мы перейдём улицу.

The verb стать often means to become. In this sentence, светофор станет зелёным literally means the traffic light will become green.

That is a very natural way to say this in Russian, because Russian often focuses on the change of state:

• красный → зелёный
• не зелёный → зелёный

So the sentence is built around the idea when the light turns green.

Because Russian is expressing a change, not just a state.

• станет зелёным = will become green / will turn green
• будет зелёным = will be green

For a traffic light, станет зелёным is more natural, because we are waiting for the moment when it changes color.

Also:

• будет зелёный is not correct here
• if you use быть, you would say будет зелёным, with the instrumental case

But even though будет зелёным is grammatically possible in some contexts, станет зелёным is the best choice here.

After verbs like стать / становиться (to become), Russian usually puts the following noun or adjective in the instrumental case.

So:

• зелёный = green
• зелёным = green (instrumental singular masculine/neuter)

Compare:

• Он стал врачом. = He became a doctor.
• Она стала счастливой. = She became happy.
• Светофор станет зелёным. = The traffic light will turn green.

So зелёным is there because it follows станет.

Because both verbs are perfective.

In Russian:

• perfective verbs form the future with one word
• imperfective verbs form the future with быть + infinitive

So:

• стать → станет = will become
• перейти → перейдём = we will cross

This is called the simple future of perfective verbs.

By contrast:

• становиться → будет становиться
• переходить → будем переходить

Those imperfective forms would not fit this sentence well, because the sentence refers to single completed future actions.

Because перейдём comes from перейти, a perfective verb, and it means we will cross as a completed action.

Here the idea is:

1. the light turns green
2. then we cross the street

So the crossing is seen as one whole event.

Compare:

• Мы перейдём улицу. = We will cross the street.
• Мы будем переходить улицу. = We will be crossing the street / we will cross habitually

The second one sounds less natural here because the sentence is not about the process of crossing, but about the resulting action.

Because перейти takes a direct object, and direct objects are usually in the accusative case.

So:

• улица = street (nominative)
• улицу = street (accusative singular)

Compare:

• Я вижу улицу. = I see the street.
• Мы перейдём улицу. = We will cross the street.

So улицу is accusative because it is the thing being crossed.

This is the classic perfective vs. imperfective difference.

• переходить улицу = to cross / to be crossing the street, with focus on process, repetition, or habit
• перейти улицу = to cross the street successfully, once, as a completed action

Examples:

• Я каждый день перехожу улицу здесь. = I cross the street here every day.
• Сейчас мы перейдём улицу. = Now we will cross the street.

In your sentence, the action is one specific future crossing, so перейдём is the natural choice.

Here it means when.

The sentence assumes that the traffic light will turn green, and then the crossing will happen. So this is a normal time relationship:

• Когда светофор станет зелёным, мы перейдём улицу.
• When the traffic light turns green, we’ll cross the street.

In some contexts, Russian когда can feel close to English whenever, but here it is clearly when.

Because Когда светофор станет зелёным is a subordinate clause, and in Russian subordinate clauses are normally separated by commas.

So:

• Когда светофор станет зелёным, мы перейдём улицу.

The comma is required.

This is true whether the subordinate clause comes first or second:

• Когда светофор станет зелёным, мы перейдём улицу.
• Мы перейдём улицу, когда светофор станет зелёным.

Yes. Russian word order is flexible.

You can also say:

• Мы перейдём улицу, когда светофор станет зелёным.

This means the same thing: We’ll cross the street when the traffic light turns green.

The version with Когда... first sounds very natural because it sets up the condition/time frame first. The version with Мы перейдём улицу... first may put slightly more focus on the main action we’ll cross the street.

Because светофор is a masculine noun in Russian. You can see that from its form and from agreement:

• зелёный светофор = green traffic light
• светофор станет зелёным = the traffic light will turn green

The adjective and verb form agree with a masculine singular noun.

It can be understood either way depending on context, but grammatically it most naturally sounds like a specific future situation:

• When the light turns green, we’ll cross the street.

That said, Russian often uses the same structure for instructions or general real-life situations too. So in the right context, it could also sound like a practical statement of what will happen in that kind of situation.

The key point is that the verbs are perfective, so the sentence presents the actions as single complete events.