"¿Cuánto cuestan las botas rojas?"
Translation:How much do the red boots cost?
A native speaker would understand you ... and think "She must be learning English." Many questions in English require the "do, does" construction. WRONG Likes he rare steak? CORRECT Does he like rare steak? WRONG Have I to pay now? CORRECT Do I have to pay now? This is a unique feature of English, whether British, American, or international. Thus, it is useful to learn and use.
I just want to add an overly complicated illustration of how the word "do" works and why it might not make intuitive sense.
In English the tense of verbs can be modified by changing the verb spelling (walk, walks, walked) but we also use auxiliaries like "will, shall, may, might, can, could, must, ought to, should, would, used to, need". The word "do" acts like an auxiliary but is often unnecessary when constructing simple sentences.
She does run - She runs
She did run - She ran
He does jump - He jumps
He did jump - He jumped
"Do" is also capable of acting like a verb but it is a place holder for the action you actually intend to "do". This creates an empty category but I won't explain that here except to say that it becomes important in the formation of more complex sentences.
"Do" can also be an intensifier in its own weird way although I'm not entirely sure the linguistic community would agree with me. For example:
I walk 5 miles every day.
I do walk 5 miles every day!
I really do. (<--this is not a proper sentence but common in speech)
(Each time the word "do" is used here it means "walk 5 miles every day")
In the example from duolingo:
"How much do the red boots cost?" the word "do" is acting in place of the verb "is" because "cost" is being used as a noun to indicate a fixed price not the act of spending.
We can reconstruct the question with the removal of "do" by asking:
"How much is the cost of the red boots?"
So why not just use "is" and be done with it?! This is probably the most interesting part of what is happening on a linguistic level. When I ask using the phrase "How much is the cost of the red boots" it sounds a little clunky and a little disinterested. Not bad at all. I may have even inquired about price this way but it does not show much of a personal interest in purchasing the product. Of if I were to even ask a salesperson "Excuse me, what is the price of this chandelier?" they would give me the price but the question indicates that I'm considering the price more than the decision to purchase the chandelier.
When I ask "How much does the chandelier cost?" I'm also indicating that I am curious about the cost for me to purchase. It's a very subtle difference but it's because "do" is capable of absorbing so many parts of the sentence that are left unspoken.This is important to note because the function of "do" in the duolingo sentence could add a personal aspect to the subject as it indicates, not always but in this example, that "you" care about the cost of the boots for personal interest and not as a matter of data collection. This indication happens intuitively for native speakers.
A more simple example of how "do" works and why we use it so much in English:
Q: How much water "do" you drink?
A: I drink a lot of water.
The same question without using "do":
Q: What is the quantity of water you consume when you drink it?
A: I consume a lot of water when I drink it.
"Do" is like the key that unlocks informal and more personal speech and is a sign of fluency even if the word "do" isn't actually -doing- anything.