The research proceeds to study the foundations of modern linguistics in Al Khalil theory which is found by the scholar of the theory Mr Abdulrahman Al Haaj Saleh. The story focuses on the opinions and points of views of the scholar in the foundations of modern linguistics and how to apply these foundations in Al Khalili theory. I followed, studied and analyzed the opinions, and then I compared everything that I reached to with the points of view and opinions of the old and modern linguistics. The research is divided into: introduction, prelude, three chapters and conclusion, where I mentioned the results that I reached to. In the prelude, I mentioned the biography of the scholar, his great achievements in the field of science and linguistics. In the first chapter 1 talked about the scientific definition of the term linguistics and the opinions of the scholars in linguistic foundations like bilingual and bilateral signifier and the signified and immediate temporal. In the second chapter generally I discussed about Al Khalil theory by giving its definition, its establishment and its location among the other theories. In the third chapter, I discussed about the signifier and the signified in Al Khalili theory, linguistic mark and how the scholar sees both these terms. Further to this, both of the terms are considered as applied sample for Al Khalili theory. Finally I concluded my research with the results that I have reached to.