Here's from Hamlet, Act 1, Scene 2. But two months dead!--nay, not so much, not two: So excellent a king; that was to this, Hyperion to a satyr; I think I understand the meaning of these sentences.| English Language & Usage Stack Exchange
The verb 'to be' is used differently in these two sentences: I am a human. I think therefore I am. In the first sentence, the verb has a subject complement ('a human'). In the second there isn't ...| English Language & Usage Stack Exchange