You did decorate your flat.

Question

And while we're on that question, the correctEnglish option, you decorated your flat, is not available. You did decorate your flat is a bizarre emphatic response to a conversation that goes something like ' Who decorated your flat?' 'We did' 'Oh, I was told it was done for you. So you did decorate your flat.' It's such an odd thing to say it's hard to construct a piece of fiction to illustrate it.

Céline Pickard · Accepted Answer

Bonjour Chris, 
You are are right that 'you did decorate' is strange. However, this particular question focuses on how to translate the causative. 'You did decorate your flat' is used as an option to trick our users a little bit and to see if you've understood the lesson and question correctly.
I hope this is helpful.
Bonne journée !