We need not go into much detail on this sentence pattern. While the adjectival subject complement modifies the subject, we might say that the nominal subject complement renames the subject. Note the examples. The second example gives us a different slant. Here we have infinitive phrases serving both as subject and subject complement.
Infinitives are flexible.
They can be used in places where we would see nominal, adjectival, or adverbial constituents. In this example, I just threw in a bunch of distracting junk to see if you could sort through it. If all this is not completely clear to you, I'm not surprised. As I said before. Try to understand thoroughly the less complicated sentences and gradually work toward understanding the more complicated ones.