Complex sentence: Curtiss and Fromkin wrote that by 1973 Genie seemed to be slowly improving her articulation and that she had clearly strengthened and gained more control over her voice, and by at least the middle of that year she could distinguish and articulate all the sounds of General American.
Split into simpler sentences: Curtiss and Fromkin wrote that by 1973 Genie seemed to be slowly improving her articulation and that she had clearly strengthened and gained more control over her voice. By at least mid- 1973, when Curtiss administered tested what sounds she could pronounce, she had the ability to distinguish and articulate all the sounds of General American.