[Grammar] (That)He is a good boy is known to me. IS "THAT" NECESSARY?

[Sahil Dhankhar]

This is not my homework.
Which of the following sentences is correct?
1- He is a good boy is known to me.
2- That he is a good boy is known to me.
3- I know he is a good boy.
4- I know that he is a good boy.
In short, kindly tell me when we can omit "that"(relative pronoun/conjunction) in a sentence.

 

[Phaedrus]

In short, kindly tell me when we can omit "the"(relative pronoun/conjunction) in a sentence.

You mean "that," not "the." You can omit it in (3) but not in (1). Sentence (1) is totally ungrammatical.

If you must begin with the "that"-clause, I suggest using topicalization rather than the passive:

That he is a good boy, I already know.

There, too, "that" can't be deleted: *"[strike]He is a good boy, I already know.[/strike]"