"IT HAS LONG BEEN KNOWN":

I didn´t look up the original reference

"WHILE IT HAS NOT BEEN POSSIBLE TO PROVIDE DEFINITE ANSWERS TO THE QUESTIONS":

An unsuccessful experiment, but I still hope to get it published.

"IT IS BELIEVED THAT":

I think.

"IT IS GENERALLY BELIEVED THAT":

A couple of other guys think so too.

"IT IS CLEAR THAT MUCH ADDITIONAL WORK WILL BE REQUIRED BEFORE A COMPLETE UNDERSTANDING OF THIS PHENOMENON OCCURS":

I don´t understand it.