The text of the card Lone Rider says:
At the beginning of the end step, if you gained 3 or more life this turn, transform Lone Rider.
Does "this turn" mean only the turn the card is cast, or at the beginning of the end step of every turn?
I am reading the card as referring to "this turn" being the turn in which this card is cast, but a friend of mine is reading "this turn" as following "the beginning of the end step" of ALL turns. The card itself I think lends itself to being probably only meaning "the turn this card is cast", because it would be I think very overpowered if "the beginning of the end step" means "at the beginning of the end step of all turns".
So my question has two parts, lastly but perhaps more importantly, in the case of ambiguous MtG card text, what's the proper way to interpret the text?