Textbook is very important in English learning and teaching. The goals of this paper are (i) to analyze the distributions of modal auxiliaries in middle school and high school English textbooks and (ii) to compare the analysis results with those of the general- purpose English corpora. Our study is based on Corpus Linguistics, and we are using corpus-analysis methods and tools in our research. In our study, we consider English textbooks as corpus texts and we will analyze them with NLPTools, which is a corpus- analyzing program. After we survey the distributions of modal auxiliaries, we will focus on a modal auxiliary can. We will examine not only syntactical distributions of a modal can but also semantic/functional distributions of this modal in English textbooks. In order to analyze the semantical distributions of can, we will use semantic tags and divide its semantic/functional category into three groups: ability, permission, and possibility. We will also divide the syntactic distributions into the nine verb phrase structures. After we closely examine the syntactic and semantic/functional distributions of a modal can, we will compare the analysis results with those of the LLC Corpus, the LOB Corpus, and the British National Corpus. Through the study, we want to show that the actual distributions and usages of a modal can in middle and high school English textbooks are a little different from those in other general-purpose English corpora.