"Я знаю де її речі."

Translation: I know where her things are.

[EmmanouilK]

I know where are her things is marked as wrong. Why?

[lambertsimnel]

The word order sounds unnatural to me. That might be the reason. If so, it's probably connected with the fact that "where are her things" is a question, but "where her things are" is something else (noun phrase?).

[sagitta145]

Yes, in a clause like this you have to use a different word order:

Where are her things? I know where her things are.

What is his favourite food? I know what his favourite food is.