The Processability Theory suggests that teachability and learn ability of a language is constrained by what learners are ready to acquire. This means that what is presented to the learners should be in line with their level and readiness. Textbooks are one of the fundamental resources of language learning and teaching, and in this sense, analyzing them is very significant for ensuring that they are appropriate for the target learner group. Accordingly, this study aims to find out whether the stages of different morphosyntactic structures in 5 English textbooks are sequenced according to the developmental stages offered by the Processability Theory and whether these 5 textbooks complement each other from the 2nd grade to 4th grade. The 5 English textbooks that are currently being used in public schools in Turkey at primary school level were selected. Textbook Analysis as a part of Document Analysis was undertaken, and the morphosyntactic structures provided in the units of these textbooks were analyzed. The findings suggest that the textbooks follow the stages suggested by the Processability Theory in general; however, there are some incompatibilities as well. The sequential development from the 2nd grade to the 4th grade is in a complementary fashion in that they tend to focus on later stages of morphological development as grade increases except for Primary School Just Fun English 3 (Tiras, 2018) which was found to present a few morphosyntactic structures that are slightly from higher stages for the expected developmental stage while most of the structures of this textbook conforms to the developmental stages of the Processability Theory as well.