The authors explored how preservice secondary mathematics teachers (PSTs) understood and applied mathematics language routines (MLRs) and related core practices during a secondary mathematics methods course focused on effective instruction for multilingual learners. The study involved eight PST participants who engaged in four cycles of learning about and implementing a core practice and focal MLR over the course of nine weeks. They present key findings addressing how PSTs comprehended the approximations of practice related to MLRs and multilingual learner core practices, highlighting both the benefits and limitations of the MLRs. Additionally, they discuss the advantages of using approximations of practice and suggest potential next steps for integrating MLRs. They conclude with implications for mathematics teacher education, emphasizing how educators can better support PSTs in teaching multilingual learners effectively.