Complex sentence: At around the same time Genie had begun to use more than one prepositional phrase in some of her sentences, and during the spring of 1975 Curtiss tested Genie on the preposition "between" and found Genie showed full comprehension in all instances.
Split into simpler sentences: At around the same time, Genie had begun to use more than one prepositional phrase in some of her sentences. During the spring of that year Curtiss began testing Genie on the preposition "between", and Genie showed full comprehension in all instances.