If I operate an inversion on (1), do I have to use the "it" or not because "Whether he met them" (see 2) looks to act as an it.
1) It is not clear whether he met them.
2) Whether he met them (it) is not clear.
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We can use clauses as Subjects in English:
In the sentences above we see a declarative clause and an interrogative clause functioning as Subjects.
However, sentences like these are not very common in English. They are awkward and difficult for listeners to process. We prefer to use extraposition constructions instead. To do this we stick a meaningless dummy Subject, the word it, in the Subject position and move the clause to the end of the sentence. In these sentences the clause now appears as a Complement of the verb instead of as a Subject. Here are more natural versions of the sentences above:
It's not surprising [that she refused your offer].
It's not important [whether you like it].
The only reason these sentences have the word it in them is because English grammar demands normal declarative sentences have Subjects. These sentences are ungrammatical without the word it:
*Is not surprising that she refused your offer. (ungrammatical)
*Is not important whether you like it. (ungrammatical)
The Original Poster's example:
[Whether he met them] is not clear.
The sentence above has a Subject, the interrogative clause Whether he met them. Because of this we don't need the word it to fill the Subject position.
However, if we move the clause to the end of the sentence to make it more natural, we will need to use the word it to fill the Subject position and make the sentence grammatical: