Complex sentence: Upon beginning to spend time with Genie, Curtiss quickly realized that Genie's language was not yet at a usefully testable level and decided to focus on getting to know Genie and gaining her friendship.
Split into simpler sentences: Upon beginning to spend time with Genie, Curtiss quickly realized that Genie's language was not yet at a usefully testable level. Therefore, Curtiss decided to focus more on getting to know Genie and gaining her friendship.