1. He broke the window on purpose. It is bad that he broke the window on purpose. I think it bad that he broke the window on purpose. I think it bad for him to have broken the window on purpose.
(Are they all grammatical?)
2. Honesty is the best policy.
It is true that honesty is the best policy. I found it true that honesty is the best policy.
(Are they all grammatical?)
3. He is deligent.
I believe that he is deligent.
I believe it that he is deligent.
I believe it for him to be deligent.
(Are they all grammatical?)
4. To help him is my duty.
It is my duty to help him.
I felt it my duty to help him.
That I help him is my duty.
It is my duty that I help him.
I feel it my duty that I help him.
I felt it my duty that I help him.
I felt it my duty that I helped him.
(Are they all grammatical?)
In #1, change "it" to "it's" after "think."
#2 is fine.
In #3, omit "it" in the 3rd sentence, and change "it for him to be" to "he is."
In #4, some are OK; others aren't.
To help him is my duty. OK
It is my duty to help him. OK
I felt it my duty to help him. OK, but awkward
That I help him is my duty. OK, but very awkward
It is my duty that I help him. OK, but awkward
I feel it my duty that I help him. "it" should be "it's"
I felt it my duty that I help him. "it" should be "it was" and "that I help" should be "to help"
I felt it my duty that I helped him. I wouldn't use this one at all.
1. Yes, all the sentences are grammatical.
2. Yes, all the sentences are grammatical.
3. Only the first sentence is grammatical. The other two sentences have incorrect word order and grammar. It should be "I believe that he is diligent."
4. Yes, all the sentences are grammatical.