"Yet" in the beginning of a sentence is similar to "But" and "Still", or a mix of these two meanings. It means, "what was already said is true, so you might expect one thing, but still it is something else". Example: He is very rich. Yet he saves every penny. == "He is very rich yet he saves every penny." So you can see there must something coming before "yet", either in the same or previous sentence. An example from Paul Simon's song "Wednesday Morning 3AM": "My life seems unreal, my crime an illusion, a scene badly written in which I must play. Yet I know as I gaze at my young love beside me, the morning is just a few hours away."