The Subject is the item that is “being held responsible” (H & M, p.117) for the validity of the argument and is identifiable by the tag question. If we take the example:
That teapot was given to your aunt, wasn’t it?
The teapot functions as both the Theme and the Subject and, as such, is unmarked. If we compare this, however, to:
That teapot the Duke gave to your aunt, didn’t he?
Here, the question is still ‘about’ the teapot, but it is the Duke who is “made to sustain the validity of the statement” (p118). Hence, the tag is “he”. We can see this responsibility in the case of certain offers and commands where the Subject is made responsible for the success of the outcome. For example, in I’ll be guided by your wishes, shall I? the speaker is not the Actor of the event but nonetheless is made to be responsible for its outcome.