Quando un pedone attraversa sulle strisce pedonali, tu devi fermarti.

Here quando has the sense of when/whenever.

In English, a traffic rule like this is often understood generally: every time that situation happens, the rule applies. Italian uses quando the same way. So this sentence is not about one specific pedestrian; it means that whenever a pedestrian is crossing at the crosswalk, you must stop.

Yes, se is also possible, but the nuance is a little different.

• Quando un pedone attraversa... = when/whenever a pedestrian crosses...
• Se un pedone attraversa... = if a pedestrian crosses...

In practice, both can work in a rule like this. Quando sounds a bit more like a regular, expected situation; se sounds a bit more like a condition. In traffic instructions, either one can appear.

Italian often uses the present tense for:

• general truths
• rules
• habitual situations
• instructions that apply whenever something happens

So un pedone attraversa is not only a pedestrian crosses in a literal present-time sense. It also means if/when a pedestrian is crossing as part of a general rule.

Likewise, devi means you must in a general, rule-based way.

Attraversa is the simple present, and it is the most natural choice for a general rule.

• attraversa = crosses / is crossing, in a broad rule-like sense
• sta attraversando = is in the act of crossing right now, with more emphasis on the ongoing action

In a traffic rule, Italian usually prefers the simple present because it sounds more general and less tied to one exact moment.

Un pedone means a pedestrian, so it refers to any pedestrian in that situation.

That works well in a rule:

• Quando un pedone attraversa... = when any pedestrian crosses...

If you used il pedone, it could sound more like:

• the pedestrian in general as a category, or
• a specific pedestrian already known from context

For this kind of sentence, un pedone is very natural.

Sulle is a contraction of:

• su = on
• le = the

So:

• su + le = sulle

Literally, sulle strisce pedonali means on the pedestrian stripes, but idiomatically it means on/at the crosswalk.

Italian often uses prepositions in ways that do not match English word-for-word, so the most important thing is to learn sulle strisce pedonali as a normal expression.

Strisce pedonali is the standard Italian term for a crosswalk or pedestrian crossing.

Literally:

• strisce = stripes
• pedonali = pedestrian

So it is literally pedestrian stripes, which makes sense because a zebra crossing is marked with white stripes.

Very often Italians simply say:

• le strisce

If the context is traffic, everyone understands that this means the crosswalk.

Yes, Italian usually drops subject pronouns when they are not needed.

So:

• devi fermarti is already enough to mean you must stop

Adding tu is possible, but it gives extra emphasis or clarity:

• ..., tu devi fermarti = ..., you must stop

In a sentence like this, tu is not wrong, but it is not required. Without special emphasis, many speakers would simply say:

• Quando un pedone attraversa sulle strisce pedonali, devi fermarti.

Because the verb needed here is fermarsi, not fermare.

• fermare = to stop something or someone
  
  • for example, fermare la macchina = to stop the car
• fermarsi = to stop oneself / to come to a stop
  
  • for example, devi fermarti = you must stop

In this sentence, the driver is the one who has to come to a stop, so Italian uses fermarsi.

Also, after a modal verb like dovere, Italian can attach the pronoun to the infinitive:

• devi fermarti

You can also say:

• ti devi fermare

Both are correct. Devi fermarti is very common and compact.

Yes, both are grammatically correct and mean the same thing here.

• devi fermarti
• ti devi fermare

This happens because with modal verbs such as dovere, potere, and volere, the pronoun can either:

• go before the conjugated verb: ti devi fermare
• attach to the infinitive: devi fermarti

In everyday Italian, both patterns are common. Many learners first notice the attached form because it is very frequent.

The comma separates the introductory quando clause from the main clause.

Structure:

• Quando un pedone attraversa sulle strisce pedonali = subordinate clause
• tu devi fermarti = main clause

In English, you also often put a comma after an introductory clause:

• When a pedestrian crosses at the crosswalk, you must stop.

So the punctuation works similarly here.

Yes. Italian word order is flexible.

For example, you could also say:

• Tu devi fermarti quando un pedone attraversa sulle strisce pedonali.
• Quando un pedone attraversa sulle strisce pedonali, devi fermarti.

Both are correct. The version with quando... first puts the condition first, which is very natural in instructions and rules. The version with tu devi fermarti first puts slightly more focus on the obligation.