"Do the police help us?"
Translation:Ĉu la polico helpas nin?
Why doesn't "la policoj" work here instead of "la polico"? Doesn't "the police" refer to officers in general?
Yes, and so does "la polico" - it's the name of the service as a whole. "La policoj" would perhaps be several police organisations together.
A single officer is a "policano" (member of the police force: polic-an-o).
How would you say in contrast to this sentence, 'Are the police helping us?' Since the two English sentences have different meanings, but are represented by the same Esperanto sentence.
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.)
I used "Ĉu la polico helpu nin?" does that work to convey "Do the police help us" ? I used this for the meaning: Do the police help us (in general) instead of "Ĉu la polico helpas nin?" which I take to mean "Are the police helping us (currently)
"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)".