"What a pretty blouse!"
Translation:¡Qué blusa tan bonita!
I think that both are correct. "tan" is used to imply a superlative. In other words, to express emphasis on how pretty the dress REALLY is.
In another lesson about pants, here is how someone explained it:
• 'Qué pantalón bonito!' • (What pretty pants!) • 'Qué pantalón tan bonito!' • (What pretty pants!) Now, if BOTH of the above mean the exact same thing, why include the word "TAN"? The TRUTH, though, is something else! "TAN" creates a SUPERLATIVE! The following are superlative: • "What REALLY pretty pants!" • "What ESPECIALLY pretty pants!" Hope this helps.
Normally we would say: (In English)This book is great. And, (In Spanish) Este libro es genial.
Now, the issue where we are having problem exists in both English and Spanish, not just Spanish. I mean, how in the world did we come up with a phrase like, "What a great book" ? If you look at it structurally, it could mean, "What? a great book? " or something else idk, but structurally speaking, it is quite a strange structure since what is usually used to ask questions
Now, the conclusion that we can draw is that in languages, there'll be regular structures that follow certain grammar rule and there will be structures that don't follow the grammar; Idioms are such. One important feature of this second kind of structure is that it cannot be translated word-to-word from one language to another(Because honestly, it cannot even be made sense in its original language if we look at the grammar and vocab)
How I looked at this issue: In english, although the structure should be "This/that/somebody's book is so great!", there's another way of saying it, which is the following: What (a) + 'adjective' + 'noun'! One advantage or disadvantage of this structure is that you shouldn't have to say "this, that, or whose, or anything of the sort".
Likewise, in Spanish: Qué + 'noun' + tan+ 'adjective' is that second structure.