Esperamos junto al bordillo hasta que el semáforo se ponga en verde.

Esperamos is the 1st person plural present indicative of esperar.

In Spanish, the present tense often covers both:

• we wait
• we are waiting

So Esperamos junto al bordillo... can naturally mean either We wait by the curb... or We are waiting by the curb..., depending on context.

Also, esperar can mean both to wait and to hope, but here the rest of the sentence makes it clear that it means to wait.

Junto a means next to, beside, or by.

So:

• junto al bordillo = next to the curb / by the curb

Here, al is the contraction of a + el:

• junto a el bordillo → junto al bordillo

A very similar expression is al lado del bordillo, which also means next to the curb.
Junto a is just a bit more compact and common in this kind of sentence.

In Spain, bordillo is the usual word for the curb or kerb.

This is a good example of regional vocabulary. In other Spanish-speaking countries, people may use different words, such as:

• cordón
• banqueta
• cuneta in some contexts, though that can also mean ditch/gutter
• sometimes people may simply rephrase it

So if you are learning Spanish from Spain, bordillo is the natural word here.

Because hasta que is followed by the subjunctive when it refers to a future event that has not happened yet.

Here, the light has not turned green yet, so Spanish uses the subjunctive:

• hasta que el semáforo se ponga en verde

Compare:

• Esperamos hasta que se ponga en verde = we wait until it turns green
• Siempre esperamos hasta que se pone en verde = we always wait until it turns green
  (habitual/general action, so the indicative can appear)

So in your sentence, the event is still pending, which is why se ponga is correct.

Because the verb here is ponerse, not just poner.

In this expression, ponerse + adjective / color / state means:

• to become
• to turn

So:

• ponerse en verde = to turn green

That is why the sentence uses:

• el semáforo se ponga en verde

Without se, poner usually means to put/place something:

• Pongo el libro en la mesa = I put the book on the table

But a traffic light does not put green; it turns green, so Spanish uses ponerse.

With this expression, Spanish normally says ponerse en + color/state.

So:

• ponerse en verde = to turn green
• ponerse en rojo = to turn red

This is especially common for traffic lights and similar changes of state.

You may also hear other ways of saying it, such as:

• cambiar a verde = to change to green

But ponerse en verde is very natural here.

Yes. Cambiar a verde is also possible and natural:

• Esperamos junto al bordillo hasta que el semáforo cambie a verde.

Both mean essentially the same thing:

• se ponga en verde
• cambie a verde

The version with ponerse is very idiomatic and common.

Yes, semáforo is a masculine noun, so it takes:

• el semáforo

Its plural is:

• los semáforos

Like many Spanish nouns ending in -o, it is masculine, though of course there are exceptions elsewhere in the language.

Yes. Spanish often uses the simple present where English might prefer the present progressive.

So:

• Esperamos junto al bordillo...

can mean:

• We wait by the curb...
• We are waiting by the curb...

If you want to emphasize the action as ongoing right now, Spanish could also say:

• Estamos esperando junto al bordillo...

But the simple present is completely normal in many contexts.

Yes, Spanish word order is fairly flexible.

The original sentence:

• Esperamos junto al bordillo hasta que el semáforo se ponga en verde.

A possible variation:

• Hasta que el semáforo se ponga en verde, esperamos junto al bordillo.

Both are grammatical. The original version sounds very natural and straightforward. Moving the hasta que clause to the front gives it slightly more emphasis.

Yes. Junto al bordillo means next to the curb, which is the natural idea here.

En el bordillo would literally suggest on the curb or at the curb itself, which is usually not what you mean if you are just waiting to cross.

So if the idea is that you are standing beside it, junto al bordillo is the better choice.