Good day to everyone! Could somebody explain me why in the following sentence "that he was disappointed" is S (subject)? (It)-S (must be confessed)-V (that he was disappointed)-S.
The classical transformational analysis is that the deep structure is [[(Someone)] [must confess [that he was disappointed]]]. This gives you "the meaning" of the sentence. This is passivized to give you [[that he was disappointed] [must be confessed (by someone)]], but you can get rid of the agent phrase. Then you extrapose the sentential subject, leaving "it" in surface subject position.
The main argument is semantic, that to get the right interpretation, the embedded S has to be the subject; also to conform to the subcategorization requirements of the verb, again, the S has to be the subject, though this is obscured by the fact that the clause is also passivized. The technical details in contemporary Minimalism is not clear to me, but I think can be inferred from the classical transformational analysis.