[pak]

"They is so smart!" --> the apparent subject-verb disagreement is just too unfamiliar to most ears.

[socksy]

You are still supposed to use plural declensions despite talking about a single person.

For instance: "if such a person existed, they were very quiet about their existence". Note that it's not "they was", which is grammatically incorrect.

[elblanco]

That's because it should read, "they are so smart".