What is the grammar -다는 것을 안다.
The sentence 영국에는 역사적 명소가 많다는 것을 안다. what does it mean when adding "다는 것"?

Does the meaning change if I removed "다"?
영국에는 역사적 명소가 많는 것을 안다

And, this sentence is it same grammar pattern as above but in contracted form?
이미 끝나 버렸단 걸 알까?

Question

What is the grammar -다는 것을 안다.
The sentence 영국에는 역사적 명소가 많다는 것을 안다. what does it mean when adding "다는 것"?

Does the meaning change if I removed "다"?
영국에는 역사적 명소가 많는 것을 안다

And, this sentence is it same grammar pattern as above but in contracted form?  
이미 끝나 버렸단 걸 알까?

Answer

V/A+다는 것 is a short form of the indirect quotation pattern V/A+다고 하는 것,
which means "the thing that is called + V/A" and it is used to express what you think the definition or the nature of a certain action/state/verb.

영국에는 역사적 명소가 많다는 것을 안다. I know that it is said that there are many famous places in England. (knowledge by indirect experience)

영국에는 역사적 명소가 많는 것을 안다(X)
영국에는 역사적 명소가 많은 것을 안다(O) I know that there are many famous places in England.(knowledge by direct experience)
많는 is wrong conjugation because 많다 is an adjective.

You are correct.
이미 끝나 버렸단 걸 알까? = 이미 끝나 버렸다는 것을 알까?

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