19th century is the century of historical comparative linguistics. 1808, Schlaege (1772- 1829) published an academic paper entitled "On Indian Language and Wisdom". He emphasized the study of the internal structure of language, pointing out that Sanskrit is closely related to Latin, Greek and Germanic languages in vocabulary and grammar, and used the term "comparative grammar" for the first time. At the beginning of Danish 1787- 1832, there were German Grimm (1785- 1863) and Baup (17965438). Lasker published a grammar book on Old Scandinavian at 18 1 1 and another grammar book on Old English at 1830. In these two books, he first compared the etymological forms of different languages by phonetic alphabet correspondence. In fact, the correspondence in the later Green's Law was first put forward by Lasker and proved by examples. Grimu's German Grammar (second edition) published in 1822, discusses letters in a large space and expounds the phonetic correspondence between German and other Indo-European languages. These phonetic correspondence laws he discovered are called "Green's Law" by later generations. The purpose of Baup's language study is to find out the original grammatical structure of the language. In his book Comparative Grammar, he declared that his aim was to make a comparative description of related languages and explore the laws governing these languages and the sources of their inflections. It was in the process of exploring the original grammatical structure that he discovered the principle of comparative grammar. Some people think that when evaluating the principle of comparative grammar, his discovery can be compared with Columbus' discovery of the new continent.