Should an infinitive be treated as a subject or object? For example, in 'I want to know it', is the 'to know' subjective or objective?
Let me give you two views on this. Part one is my preference.
Generally, infinitive can act like an adverb, adjective, and a noun in a sentence. As a noun (or a noun substitute), an infinitive can function as the complement, subject or the object of the sentence.
As the subject:
To go, even after all that trouble, didn’t seem worthwhile anymore.
To err is human.
As the complement:
My goal is to write.
And finally the object for which you gave an example:
He wants to see.
Here to see is the object. To see is generally what he desires.
In your example to know it is what I desire and then is the object of want.
"to know it" in your example is a complement. According to Cliff, complements are Noun Clauses (i.e, clauses used as nouns), and they may function either as Subject or as Direct Object. Complements are of different kinds one of which is infinitive clauses. The other two are gerund and that clases.
Based on Cliff's classification. "To know it" is a non-finite infinitive clause which is the object of the sentence.
However, in either case, it's the object in your example.