Clause with how

Question

1. I don't know how difficult it is to learn a new language

2. I don't know how difficult to learn a new language is

3. I don't know how difficult learning a new language is

are these sentences grammatically correct?

Answer 1

Let's make the snetences a bit simpler. Consider the following examples:

I know how hard it is to learn English

I know how hard learning English is.

Based on the above examples, number 1 and 3 are grammatical, but 2 is not. Personally, I prefer number 1 to number 3.