[Marazan]

So would saying "you is".

You are an idiot. They are an idiot

Thou is an idiot if you want to be 'correct'.

[smudgymcscmudge]

I'm talking about third-person pronoun, not second. Sentences like "They is in the office".

[Marazan]

You'd say "They are" just like you say "You are" not "You is" for second person. English isn't some precisely specified 1 look ahead formal grammar.