This study tests the Fundamental Difference Hypothesis, which posits that adults, unlike children, rely on explicit, problem-solving mechanisms to learn a second language, and thus, only those with high verbal analytical abilities can achieve near-native competence. The study, conducted with 57 adult Hungarian-speaking immigrants, confirmed the hypothesis by showing that few adult immigrants scored within the range of child arrivals on a grammaticality judgment test, and those who did had high verbal analytical abilities. This ability was not a significant predictor for childhood arrivals. The findings replicate Johnson and Newport's (1989) study and provide an explanation for its apparent exceptions. The study also explores the role of verbal aptitude in second language acquisition, finding that only adults with high verbal aptitude could achieve near-native competence. The results suggest that the critical period effect is pervasive in second language acquisition, particularly in morphosyntactic competence, but may not be visible for advanced learners. The study concludes that the critical period effect is robust and that the role of verbal aptitude is crucial for successful second language learning.This study tests the Fundamental Difference Hypothesis, which posits that adults, unlike children, rely on explicit, problem-solving mechanisms to learn a second language, and thus, only those with high verbal analytical abilities can achieve near-native competence. The study, conducted with 57 adult Hungarian-speaking immigrants, confirmed the hypothesis by showing that few adult immigrants scored within the range of child arrivals on a grammaticality judgment test, and those who did had high verbal analytical abilities. This ability was not a significant predictor for childhood arrivals. The findings replicate Johnson and Newport's (1989) study and provide an explanation for its apparent exceptions. The study also explores the role of verbal aptitude in second language acquisition, finding that only adults with high verbal aptitude could achieve near-native competence. The results suggest that the critical period effect is pervasive in second language acquisition, particularly in morphosyntactic competence, but may not be visible for advanced learners. The study concludes that the critical period effect is robust and that the role of verbal aptitude is crucial for successful second language learning.