Noun-clause beginning with "what" involving an infinitive [closed]

Solution 1:

According to my understanding, both sentences (2c, 2c') are grammatical and express two different notions. I make out the respective meanings to be as follows.

• We have not learned what is important to learn from the past.
  (The things are important from the point of view of learning; the focus is on the things. "What" is the subject of "is".)

We haven't learned what are the things in the past that it should be important to learn, we do not know whether there are any (because we couldn't study that matter or because there is no way one can come to a conclusion, etc.).

• We have not learned what it is important to learn from the past.
  (The learning of the things is important; the focus is on the recognized fact that this type of learning is important. "What" is the object of "learn" ('it is important to learn "what" ').)

"It" is impersonal "it". We know that(, most likely,) there are important things to learn from the past, without knowing precisely which, or we might even know which, nevertheless, whether the former or the latter, we haven't learned any.

By the knowledge of God, I understand that by which we not only conceive that there is some God, but also apprehend what it is for our interest, and conducive to his glory, what, in short, it is befitting to know concerning him.

A simplification that changes nothing to the syntax and semantics of what is being retained (so as to see clearer through the maze)

By the knowledge of God, I understand that by which we apprehend what it is for our interest to know concerning him.

"It" is impersonal "it". We know the facts that are for our interest to know and I apprehend (perceive with the intellect) them; if "it" were not used, then, the meaning would be "whatever the facts that are in our interest to know I apprehend them.

The comment could be made that in the present context of apprehension of admittedly well know facts towards a definition of the knowledge of God there is the implication of a definition of what is already defined, therefore some sort of fallacy (defining the defined). Therefore, it is likely that "it" should not be used in this sentence.