why long to-infinitive is placed at the back.

I've always thought why long to-infinitive is placed at the back is English hates an improper fraction like a person whose head is so big that he can easily fall down. Is that the reason?

ex)It is hard to understand Korean grammar.(0)
To understand Korean grammar is hard.(x)

keannu said:

I've always thought why long to-infinitive is placed at the back is English hates an improper fraction like a person whose head is so big that he can easily fall down. Is that the reason?

ex)It is hard to understand Korean grammar.(0)
To understand Korean grammar is hard.(x)

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There is nothing wrong with the second sentence.

But can you explain why you prefer to use "it ....to..." instead of "to...is ...", especially when to infinitive is long?
I might have said wrong "To understand Korean grammar is hard." is never wrong, but in case of "To understand Korean grammar to the perfect level without any trace of flaws is hard", it would have to be amended to "It is hard to....".

keannu said:

But can you explain why you prefer to use "it ....to..." instead of "to...is ...", especially when to infinitive is long?
I might have said wrong "To understand Korean grammar is hard." is never wrong, but in case of "To understand Korean grammar to the perfect level without any trace of flaws is hard", it would have to be amended to "It is hard to....".

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In English, we like to get to the verb sooner rather than later. I find it much easier to concentrate on the details of such a long subject if I know what's going to happen to it.
But this is obviously not necessary in a language, since some, such as German, put the verb at the end.

This is just a guess, but I believe this is because putting the verb first allows you the freedom to keep expounding on the idea. I would intuitively start with "It is hard" because it fixes the action and then I am free to think on my feet and add meaning to the sentence as I go.

It is hard to understand.
It is hard to understand Korean.
It is hard to understand Korean grammar.
It is hard to understand Korean grammar to the perfect level.

My $0.02