"Do the police help us?"
Translation:Ĉu la polico helpas nin?
Esperanto doesn't make all the distinctions that English does - just because two sentences are different in English doesn't mean that Esperanto has to make a distinction. (In this case, you can make the distinction, by using "estas helpanta" for the continuous, but it's not often done.)
"Ĉu la polico helpu nin?" would be "Shall the police help us?" or "Should the police help us?" -- not "Do the police help us?".
I'd imagine you might say it if you are in a situation where you are considering asking the police for help and are asking a friend whether he agrees that their help would be a good idea.
Compare "Ĉu mi helpu vin?" ("Shall I help you?") which you might ask your child who is struggling with its homework.
"Ĉu la polico helpas nin?" can mean either "Do they help us? (in general)" or "Are they helping us? (right now)".