Scaffolds establish a cognitive connection with the students and what they want to express. Supporting the collaborative knowledge building process with scaffolds is crucial for the participation and continuity in the online discussions. In this research, where a quasi-experimental design is used, the contributions of the students in the online collaborative knowledge building process are examined in terms of role assignment, sentence opener scaffolds, and self-determination. 77 teacher candidates, who are registered to Computer II course, are assigned to 4 groups, in three of which scaffolds are used, and in the remaining one of which scaffolds are not used. The students contribute to the knowledge building process in the first group by using the sentence openers, in the second group by being assigned with roles, in the third group by both being assigned with roles, and using the sentence openers appertaining to the respective roles, and in the fourth group by not making use of any scaffold. Using content analysis and MANOVA, the research results reveal that using scaffolds, especially the combination of sentence openers and role assignment scaffolds encouraged higher cognitive levels of knowledge building. Significant differences with high effects were found between the groups for the dimensions of self-determination: self-awareness and perceived choice. The research points out some suggestions for future research.