Short answer (tl;dr)
In I know [what freedom is], the interrogative phrase what occurs at the beginning of the subordinate clause because the grammar says the interrogative must be the first word 1(or phrase) in the clause. The word freedom is the Subject of the verb BE and therefore occurs directly before it. We thus get an item intervening between the Wh-word and the verb.
In clauses such as I know [who is coming], the who is not only the interrogative word, but also the Subject of the verb BE. For this reason, the wh-word occurs directly before the verb. The word coming is the Complement of the verb BE and occurs after it.
1For the remainder I use 'interrogative word' as shorthand for 'interrogative word or phrase'.
The Full Story
- Correct: I know what freedom is.
- Wrong: I know what is freedom.
The Original Poster notes that the first example seems natural and the second less so. In actual fact the second example is grammatical but is a slightly different question, and a slightly unusual one. We'll set that aside here and ask instead why the first, natural, example is different from the Original poster's other ones below:
- I know who is coming.
- I know which is better.
- I know what is next.
The difficulty that arises with the original poster's example, and which doesn't apply to examples (1-3), is that the Original Poster's example has two noun phrases (NPs) joined by the verb BE. One of these is the Subject and the other is the Predicative Complement.
In a declarative clause it is easy to tell a Predicative Complement NP from a Subject one. The Subject comes first and the PC comes after the verb. Consider the following:
- Bob is Hamlet
- The man with the hat is Bob.
I have bolded the subjects in each case. The PC in (4) is Hamlet, in (5) it is Bob. The normal way to understand (4), is that the speaker is telling the listener what role Bob has in a play (Hamlet is a character from a Shakespeare play). The normal way to understand (5), if we were listening in, is that the listener can see the man with the hat but is unaware of his identity, or that they have heard of Bob, but don't know who he actually is.
Now, let's imagine we want to know who's playing the role of Hamlet, or what the identity of the man with the hat is. We could convert (4) and (5) into questions thus:
- Who is Hamlet?
- Who is the man with the hat?
Now if you look at (6) and (7), it would be easy to think they have the same structure. They both have Who as the initial question word, and the verb BE as a copular verb followed by another noun phrase. However, if you look at the bolded NPs, which are the Subjects of the two examples, you will see that the structure is actually very different, even though the two sentences look identical. The structure of (6) is: Subject, BE, PC. The structure of (7) is PC, BE, Subject.
The word order in (6) is the same as the word order in (4). When the wh-word in a question is the Subject of the clause, the word order remains the same. However, in (7), the wh-word, which appears at the front of the clause, is the Predicative Complement. When the initial wh-word is not the Subject (or part of it), we see Subject-auxiliary inversion, and the initial tensed verb must appear before the Subject. In (7), we see that not only has the PC Who moved to the front of the clause, but the verb is has moved to a position in front of the Subject, namely the man with the hat.
We can see, then, that the phrase order in such interrogative clauses where we see two NPs conjoined by BE, is highly ambiguous.
However, this is caused by Subject auxiliary inversion occurring in the example where the PC moves to the front of the clause. Subject-auxiliary inversion does not happen in subordinate interrogative clauses. In subordinate interrogative content clauses, the wh-word or phrase moves to the front of the clause, but the other phrases stay in the same positions they would have in a declarative one. Embedding (6) and (7) in a larger clause will make the two structures clear again:
- Do you know who is Hamlet?
- Do you know who the man with the hat is?
We intuitively understand that who is the subject of (8) and The man with the hat the subject of (9) because the two phrases occur in their normal positions directly in front of the verb BE.
The Original Poster's examples
In I know [what freedom is], the word freedom occurs before the verb BE because it is the Subject of the clause. The Predicative Complement of the clause what appears at the front of the subordinate clause because it is an interrogative word.
In examples (1-3), the Subject of the verb BE is clearly the word who, as the phrases that follow are not NPs but verbs and adjectives and so forth. Also, they occur after the verb BE! The Who is at the front of the clause as much because it is the Subject as because it is the question word.
Lastly, the OP asks why they cannot construct questions like those shown in (1-3) with the interrogative words when, where, why, whether and if. The answer is that it is impossible to do this with the interrogative words whether and if because these two words are both subordinators. They just sit at the front of the clause and tell you that it's a question. They can therefore never be the Subjects of the subordinate clauses they occur in front of. With the interrogative words when, how it is a different story. It is difficult to use them in clauses such as (1-3), but not impossible. This is because it difficult but not impossible to use them as Subjects. When is a temporal preposition (adverb in old-fashioned grammar) and how is either an adverb of degree or of manner. Both of these are difficult to use as subjects. But not impossible:
- I know when is best to have the party.
- I know how big a donation is required.
That's all folks!
