The variation you're having trouble understanding is notorious. Speakers differ on how they interpret the syntax here:
- It is [you who are nice]. / You are nice.
- 'are' agrees with 'you'
- It is I who am nice. / I am nice.
- It is he who is nice. / He is nice.
- It is we who are nice. / We are nice.
- It is they who are nice. / They are nice.
- It is you [who is nice]. / ...who is nice.
- 'is' agrees with 'who', which is by default singular in number.
- It is me who is nice. / ...who is nice.
- It is you who is nice./ ...who is nice.
- It is him who is nice. / ...who is nice.
- It is us who is nice. / ...who is nice.
- It is them who is nice. /...who is nice.
- Descriptively, the pattern we see is this. If the pronoun is in its object form (me, you, him, us, them), the verb is in third person singular (See also Akmajian (1970)).
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