Knowing the sentence structures (i.e., information that guides the assembly of words into sentences) is crucial in language knowledge. This knowledge must be stable for successful communication, but when learning another language that uses different structures, speakers must adjust their structural knowledge. Here, we examine how newly acquired second language (L2) knowledge influences first language (L1) structure knowledge. We compared two groups of Korean speakers: Korean-immersed speakers living in Korea (with little English exposure) versus English-immersed speakers who acquired English late and were living in the US (with more English exposure). We used acceptability judgment and sentence production tasks on Korean sentences in English and Korean word orders. Results suggest that acceptability and structural usage in L1 change after exposure to L2, but not in a way that matches L2 structures. Instead, L2 exposure might lead to increased difficulties in the selection and retrieval of word orders while using L1.