Complex sentence: Isaac Asimov said that, from the time of his first story, it was accepted that Heinlein was the best science fiction writer in existence, adding that he would hold this title through his lifetime.
Split into simpler sentences: Isaac Asimov said that, from the time of his first story, it was accepted that Heinlein was the best science fiction writer in existence. He said that he would hold this title through his lifetime.