[Silverleaf]

Heidegger does a lot to try to explain why being and why not being (especially in Being and Time, aka Sein und Zeit, but if you want to tackle that bitch do yourself a favor and pick up "A History of the Concept of Time"; it's kind of like the lecture notes to Sein und Zeit). In brief, I'll mention the concept of presences and absences, which is pretty essential to phenomenology.

I have a pack of cigarettes. I hold it up and look at it, and I know it's a pack of cigarettes -- even though I can't see the whole object (there is another side that I cannot see, though you might be able to see if you are standing in the correct spatiotemporal location), through it's manifold appearance (the numerous sides, aspects, and profiles of the object), I know it's a pack of cigarettes. It's a question of what is present and what is absent. The absence is an essential part of the presence of the object -- it is PART of the object.

I think the same can be applied to existence. If this, then there is that. If exist, then does not exist.