"What do you think of this dress?" is definitely the same as "What do you think about this dress?"
"What do you think of that dress" is accepted. I'm surprised Duolingo is accepting any answer with "this" as a translation for "desse", being that they don't accept that slight change in most questions. Don't get me wrong, in speech we often use them interchangeably, but here they are usually more strict about it here. =)
I think that if "desse" is opposed to "deste", then "desse" is
should be translated as "that", not as "this".
why is "desse" this in this sentence? It is usually translated as "that".
Acha/pensa...what's the difference and how do you know what's appropriate to use
Achar = to find, to think, to have an opinion Pensar = to think Honestly, in this context, the opinion of someone, they mean the same. But you can't extrapolate and say something like "os grandes achadores da Grécia" (the great thinkers of Greece), in this case, only the word "pensadores" is right. When you're talking about their ideas is ok to use "achar" - What did Plato think about love? / O que Platão achava do amor? / O que Platão pensava sobre o amor? When you mean "to find an object or a person", only use "achar".