The original German is (from the Introduction, par. 2):
Hinsehen auf, Verstehen und Begreifen von, Wählen, Zugang zu sind
konstitutive Verhaltungen des Fragens und so selbst Seinsmodi eines
bestimmten Seienden, des Seienden, das wir, die Fragenden, je selbst
sind. Ausarbeitung der Seinsfrage besagt demnach: Durchsichtigmachen
eines Seienden – des fragenden – in seinem Sein.
Das Fragen dieser Frage ist als Seinsmodus eines Seienden selbst von
dem her wesenhaft bestimmt, wonach in ihm gefragt ist – vom Sein.
In English, this easily gets confusing as Seiende and Sein, which are clearly distinctive in German, are translated as 'being'. The translation is sometimes improved be writing Sein as Being:
Regarding, understanding, grasping, choosing and gaining access to,
are constitutative attitudes of inquiry and are thus themselves modes
of Being [Seinsmodi] of a particular being [Seiende], of the being
[Seiende] we enquirers ourselves in each case are. Thus to work out
the question of Being [Sein] is to make a being [Seiende] - one who
questions - transparent in its Being [Sein].
Asking this question, as a mode of Being [Seinsmodi] of this being
[Seiende], is essentially determined by what is it asked about in it -
Your question actually falls into two:
- which different senses Heidegger assumes he uses;
- which different senses he actually uses.
Ad 1. The answer clearly: he distinguishes between being (Seiende) and Being (Sein). Simply put, Seiende/being is a thing in the world, like rocks or humans. Sein/Being is the way a particular being is. In traditional metaphysics these have more familiar names such as:
- Seiende/being: thing, particular
- Sein/Being: idea, ousia, form, essence, representation, conception, universal
Heidegger does not use these names, as he thinks these names are conceived from one specific way of Being (Vorhandenheit), which has been misapplied to all beings (esp. humans).
Ad 2. Perhaps Heidegger actually confused several meanings of Sein/Being into one, as is argued for example by Herman Philipse (in his Heidegger's Philosophy of Being: A Critical Interpretation). So the answer is debatable.