Complex sentence: In January 1974 the scientists noticed the first instances of her using copulas in her spontaneous sentences, which they considered a significant gain, but Curtiss noted that Genie never used a contractible copula such as "That's mine".
Split into simpler sentences: In January 1974 the scientists noticed the first instances of her using copulas in her spontaneous sentences, which they considered a significant gain. However, Curtiss noted that Genie never used a contractible copula such as "That's mine".