Můžeš přejít ulici až u semaforu, ne na rohu.

In this sentence, až means something like only or not until.

So Můžeš přejít ulici až u semaforu means:

• You can cross the street only at the traffic light
• literally, You can cross the street not until you get to the traffic light

This use of až is very common in Czech. It often marks a limit or the first point where something becomes possible.

Because ulici is in the accusative case, which is used for the direct object.

The verb přejít means to cross, and the thing being crossed is the direct object:

• přejít co? → ulici

The noun ulice is feminine, and its accusative singular form is ulici.

So:

• nominative: ulice = street
• accusative: ulici = street (as the object of crossing)

U takes the genitive case, so semafor becomes semaforu.

• u semaforu = at/by the traffic light

This is a location phrase. It does not mean standing literally on the traffic light, but rather near it / at that point.

So:

• u + genitive
• semafor → semaforu

Because u is the natural preposition for by / at / near something like a traffic light.

• u semaforu = at the traffic light
• na semaforu would sound wrong here, because na usually means on a surface or in some special idiomatic expressions

A traffic light is treated as a point/location you are near, not a surface you are on.

This is a question of aspect.

• přejít = perfective → to cross, to get across, a completed action
• přecházet = imperfective → to be crossing / to cross habitually / repeatedly

In this sentence, přejít fits because it refers to a single completed action:

• Můžeš přejít ulici... = You can cross the street...

If you used přecházet, it would sound more like a general repeated activity or process.

It can suggest both, depending on context.

Můžeš literally means you can, but in real usage it can express:

• ability: you are able to
• permission: you are allowed to
• practical possibility: it is possible/acceptable to

In this sentence, it most likely means you are allowed to / you should only cross there.

So the sense is close to:

• You may cross the street only at the traffic light, not on the corner.

Czech often leaves out words that are easy to understand from context.

• ne na rohu = not on the corner
• a fuller version could be ale ne na rohu = but not on the corner

Both are possible, but the shorter version sounds natural and efficient.

The contrast is understood automatically:

• at the traffic light, not on the corner

Because Czech uses na rohu for on the corner / at the corner.

This is just the normal idiomatic expression:

• na rohu = on the corner
• u rohu would usually not be the normal choice here

So even though English says on the corner, Czech also uses na here in a very similar way.

Yes. Czech word order is flexible, though some versions sound more natural depending on emphasis.

The original:

• Můžeš přejít ulici až u semaforu, ne na rohu.

This is natural and clear.

You could also hear:

• Ulici můžeš přejít až u semaforu, ne na rohu.
• Můžeš ulici přejít až u semaforu, ne na rohu.

These are all understandable, but the original sounds very normal. Word order in Czech often changes to shift focus or emphasis rather than basic meaning.

Yes, you may hear both, but they are not used in exactly the same way.

• přejít ulici = cross the street
• přejít přes ulici = cross over the street / go across the street

With přejít, the direct object without a preposition is very common and natural:

• přejít ulici

Adding přes is also possible in some contexts, but here přejít ulici is the most straightforward wording.

Not necessarily a zebra crossing by itself, but it strongly suggests a place controlled by traffic lights.

• u semaforu = at the traffic light
• so the idea is: cross there, not at the corner

In real life, this often implies a designated crossing point near the lights, even if the sentence does not explicitly mention přechod (crosswalk).

Because Czech does not have articles like the and a/an.

So:

• ulici can mean the street or a street
• u semaforu can mean at the traffic light or at a traffic light

The exact meaning comes from context. In this sentence, English naturally uses the because the speaker probably means a specific street corner and a specific traffic light already known in the situation.

Yes. Až adds a limiting sense.

Compare:

• Můžeš přejít ulici u semaforu.
  = You can cross the street at the traffic light.

• Můžeš přejít ulici až u semaforu.
  = You can cross the street only at the traffic light / not before that point.

So až is important because it emphasizes that crossing earlier, for example at the corner, is not allowed or not appropriate.