"It is they who lied" or "it is them who lied?"
Which is the correct usage of the third person, plural pronoun?
It is they who lied.
It is them who lied.
The subordinate clause "who lied" is a red herring in this example. By that, I mean that "who lied" has no effect on the case of the main clause.
In "who lied", who is getting nominative case. That is why, if you make a who/whom distinction, you would say:
- It is John who lied. (nominative)
- It is John whom I lied to. (accusative)
So, who is bearing the burden of the case of the subordinate clause, which means your question is actually:
"Which is correct: 'It is they' or 'It is them'?"
or, even more to the point:
"Does English use nominative case on both sides of the copula?"
For this, the answer is not completely straightforward. But here is my (hopefully objective) take:
Usage indicates that a clear majority of people prefer to use the accusative case ("it is them"), and there is other evidence to indicate that the accusative is the default case in English. If you look at speech, you will find that the preference is even clearer; in writing, the preference is not as strong but still quite clearly in favor of "it is them".
There is also clear evidence that a large-enough minority prefers "it is they" that both are legitimate uses. Certainly, both constructions are linguistically sound variants, even if the accusative is the default case. (The default case thing is more of an explanation for why accusative in the copula should ever happen.)
There are some who believe that "it is they" is the objectively correct answer. In an approach similar to this answer about "data is" vs. "data are", I would suggest that it is important to be aware of your audience and your group. Whether a construction's usage is in the minority or not, certain audiences in certain contexts expect certain language. This means that you might be judged for not using "it is they" in these groups. It's not really a matter of objective correctness, but rather, group membership. Just as choosing semantically correct words and pronouncing them correctly are important in your communication, the register and language you choose to use within a group is also important. (You may choose to intentionally label yourself as an outsider, but the important thing is to be aware that you can make that choice with the words you use!)
So, the short answer is: both versions exist, so do what feels natural to you, but consider your audience expectations.
It's they.
Who lied? They did. → It is they who lied.
Edit: them continues to sound Just WrongTM to me, but knowledgeable folks who I respect tell me that it is also acceptable, maybe even preferred in some contexts (e.g. spoken language). So I'm officially confused.