The purpose of the study is to evaluate how prospective mathematics teachers (PMTs) modify tasks to facilitate students' learning of pattern generalization through the use of their mathematical knowledge for teaching. Case study, which is a type of qualitative research method, was used to determine the mathematical characteristics that PMTs use when modifying a mathematical task. The knowledge from which PMTs draw to modify the task has also been outlined. Accordingly, data were collected from PMTs' task modifications and reflection reports. When PMTs worked on two or more forms of modification, as compared to just using one type of modification, they modified tasks more effectively and comprehensively. The PMTs who make condition modifications need to utilize specialized content knowledge through the use of models or tables. They aimed to help middle school students understand using these modifications, and thus they also utilized their knowledge of content and students. They also used their knowledge of content and teaching, especially while making modifications to questions and context. Task modification activities can be used to help prospective teachers notice the mathematical and pedagogical affordances and limitations offered by tasks.