That would be "Tyto šaty". The course insists on the difference between "The dress" and "this dress".
Well, in English there's a difference between "The dresses are long" and "The dress is long". I suppose in Czech it depends on context... Thanks for your reply!
Jak poznáte množné číslo z TY ŠATY jsou dlouhé The dress is long ÷ the dresses are long