Learn Portuguese (Portugal): O sinal está amarelo, não corras!

In Portuguese, you cannot just put não in front of the normal present tense to tell someone not to do something. For the informal you (tu), the negative imperative uses the Present Subjunctive form. This means ER verbs like correr swap their normal ending for an 'a' or 'as'. So, tu corres (you run) becomes não corras! (don't run!).

How would I say 'don't run' formally to someone I just met?

For formal situations using você, you drop the 's' from the informal negative command. ER verbs take an 'a' ending in the formal imperative. So, you would say não corra! instead of não corras!.

Why do we use está instead of é for the color yellow here?

We use the verb estar because the light being yellow is a temporary state — it will change to red in a few seconds! If you said o sinal é amarelo using the verb ser, it would mean the physical object itself is permanently painted yellow.

I thought a traffic light was a semáforo. Is sinal also correct?

Yes! While semáforo is the exact technical word for a traffic light, it is very common in everyday Portuguese to just call it o sinal (the signal or sign) when talking about driving or crossing the street.