A prepositional phrase is such a phrase which begins with a preposition (on, under, over, etc) and ends with a noun, pronoun, gerund, phrase etc.
So here, the correct sentence will be "On the stage the members of the orchestra began to tune their instruments".
The French and Indian war ended in 1763.
The British wanted to appease the native americans.
This proclamation checked the encroachment of settlers on Indian territory.
Since this was issued it has because a part of Native American law in the US and Canada.
