"There were two balls on the playing field."

Translation: Na hřišti byly dva míče.

[alturing]

Why is it "byly" and not "byli", when "míč" is masculine?

[VladaFu]

It is masculine inanimate. Only masculine animate have "byli".

[fehrerdef]

what about "Byly na hřišti dva míče."?

[VladaFu]

As an introduction to a fairy tale perhaps, otherwise no, that is a question word order.

[fehrerdef]

:-)

And what about "byly dva míče na hříšti"

[VladaFu]

The same. It is not impossible, but normally it is very unnatural unless it is the first sentence of a story (typically a fairy tale) you are telling.