Sentence structure

#2 is natural - although not necessarily true.
#1 is not natural. The reason for the difficulty is not given.

lucas21cIt is difficult for adults to learn a new language.

This is the one you need. Here's the pattern:

It is [difficult / hard / easy / possible / impossible / important / essential / trivial / ...] FOR ... TO ....

for goes with to in these expressions. They are all natural and commonly used.

CJ

Then, how about 'It is impossible that people over 55 years old learn a new language (which is not incestuous with their mother tongues)?" Does it also sound less natural than "It is impossible for people over 55 years old to learn a new language (which is not incestuous with their mother tongues)?"

lucas21chow about 'It is impossible that people over 55 years old learn a new language ...

It doesn't sound right to me. Where did you get it?

lucas21cDoes it also sound less natural than "It is impossible for people over 55 years old to learn a new language ...

Yes. This one sounds better.

CJ

1. I extracted it from an English teaching blog. And, there are another sentence for the usage of "it is impossible that ~" like the following one.

It is impossible that they did not see it.

Are both of "It is impossible that people over 55 years old learn a new language" and "It is impossible that they did not see it" grammatically and semantically wrong? Or

lucas21cIt is impossible that they did not see it.

With a present-tense clause and a that-clause in the past tense, this makes more sense, because you can't show two different tenses if one clause is expressed as a for ... to ... structure (unless you use to have + p.p. — a structure which you asked about below).

It is i