'Das' is the object here; the subject is 'du'. Putting the object first like this is fine, as long as the verb comes second.
It's a way to emphasize the object, like how in the English version we would normally give the word 'that' extra stress ("you can say THAT again").
Yeah but then again, "puoi dirlo forte" is "you can say it loud" otherwise you would have to say "puoi dirlo di nuovo" or "puoi ripeterlo", which would sound very clumsy. And as for loudly instead of loud, yes it would be grammatically proper being it an adverb and not an adjective but I believe nobody would say it today.
Here in the UK, people definitely do say "loudly". It is still considered incorrect to use adjectives like loud, quick, heavy, high, complete and short as adverbs.
used for expressing strong agreement with what someone has said
‘This is so boring!’ ‘You can say that again!’
Because you would miss the point, which is that this is how this phrase is used by real people: not whether it is grammatically correct. This is German not English and when they say it they mean something other than the dictionary version.
So you lose a point in a million point game. That's how people learn by making mistakes.
Hmm. Well, in a learning space devoid of real people, using what you already know - grammar and vocab - is all we have. Anything other than that is a shot in the dark. And because it's idiom, you don't actually learn it because idiom works with memorization and practice.
Additionally, in the iOS apps, you're only allowed to be wrong five times a day. So you burn one not knowing that this idiom. Then later, you forget what the actual idiom is, or mix it up, and boom, you lose another. So that's two chances blown "learning" idiom and you have 10 more sentences ahead of you to finish your lesson. This is not a competition. It's supposed to be learning. Any gamification should be limited to keeping you interested in practicing and not penalizing you for learning.
"That you can say again" as opposed to "you can say that again". Because "das" is at the beginning, I am inclined to say "that you can say again"; however, Duo will not accept this. If this is a idiom, would it be proper to say "Kannst du laut sagen"? The word order is throwing me off on this one...
You're thinking about this the wrong way. German is more flexible with the location of the subject of the sentence. It can come before or after the verb. These two sentences mean the same thing, they just have slightly different emphasis:
Das kannst du laut sagen.
Du kannst das laut sagen.
The difference here is that when the object (das) is placed before the verb, it is given more emphasis. i.e. "You can say that again" instead of "You can say that again (if you want...)".
Remember, not all things have a 1-to-1 translation between any two languages.