What is the difference between Sang and Si in a sentence?
Sang is much more formal. In this case, the raja is explicitly more important than the dancer.
The pronunciation is supposed to be "sang" not "sɛng"
When are these articles used, as opposed to omitting them?
I tried using "The king summons the dancers" and it was marked incorrect, is "si" only for the singular?
Yes, "si" is only used in singular.