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Hello, everyone.

I have a question about gerunds and to-infinitives as subjects. Would you take a look at the following sentences?

1. Understanding this lesson is easy. (O) :right
1-2 To understanding this lesson is easy(O)
2. It is easy understading this lesson.(X) :wrong
2-2 It is easy to understanding this lesson.(O)
3. It is not easy understading this lesson.(?)

4-1. Protesting the new politics would be no use.(O)
4-2 To protesting the new politics would be no use.(?)
5-1 It would be no use Protesting the new politics.(O)
5-2 It would be no use to protesting the new politics.(?)

As seen, what I am wondering about is when we can put genrunds or to-infinitives in the back part of sentences. The sentences tagged with the question marks are what I don't know whether they are right or not. As the others come from a grammar book, I guess the values of them are correct. If not, please correct them and me.

P.S According to dictionaries, we can use both to infintive and gerund, while the adjective worth only allow gerund.

It might be worthwhile recalling/to recall a few important facts.
The book is definitely worth reading.
It is is definitely worth reading the book.

Is there some nuance in each adjective to need gerund or not to get along with to-infinitive? If so, what is that?

Many thanks in advance.
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. To understanding this lesson is easy.
It is easy to understanding this lesson.
To protesting the new politics would be no use.
It would be no use to protesting the new politics.

These re not to- infinitives, and they are not correct.
Comments  
I'm sorry that are all mistypoes. I have corrected them below.

3. It is not easy understading this lesson.(?)
4-2 To protest the new politics would be no use.(?)
5-2 It would be no use to protest the new politics.(?)