In this paper, with looking at the various sides where Toegye Yi Whang(退溪 李滉, 1501~1570) managed the letters properly, I would like to propose a Precise Criterion[正法] of sentences which kept the correct rules of the sentence while preserving the smoothness of the characters. For this purpose, I intend to research the inner and outer meaning of the letters, and draw the spirit of the scholar who endured the correction constantly for the best sentence as a whole by illuminating the Usage of the words and the differentiation of their meanings according to the subject. Toegye stated that the writings should be clear, precise, and worthy. He did not hesitate to fix the wrong meaning of each word that made up the sentence, if he thought that even a word was not accurate or correct at all. There is a slight amount of correction to the article, so it is not until after it has been repaired repeatedly that it will become even better and there will be no doubt to be passed on to future generations. He said that one should continue to repeatedly correct the articles without hurrying to print them. This is because if one does not fix the wrong places and the awkward places, there are many places where the sentence does not make sense. Also, he suggested that if one publishes a book, without hurrying to finish the book, it is good to publish it in perfect condition after slowly resigned and corrected. So he said that each letter should follow the meaning of a Chinese character and pursue its meaning in each passage, the sentence fits its purpose, and the meaning of the word is brief. One should not use character that has no a history, and then it could be not until the deletion is deleted that the sentences of the upper and lower sentences become smooth. He also stressed the necessity of commentary, emphasizing the correct rules of sentence. Especially, when the sentence is difficult to read, the comment should be added during the comment to remove the doubt and uncertainty to the sentences. To this end, I explained the Surplus Words (剩語) and the Reduplicate Words (疊語) in which Toegye strictly operated the characters. Toegye thought that the reason why the old people studied for a lifetime but rather lacked time was because it was difficult to understand the implicated meaning of the words and reveal their meanings by the distinction to Major & Minor and Deep & Low. So to grasp the meaning, he first proposed to understand the intention of the original meaning. That is, he proposed to understand the inner meaning of language, not to be caught in the superficial meaning of the language, the outer meaning. Toegye emphasized that the meaning of the sentence in the writing is intentional one and the truth in the writing is a criteria of the sentence, so the meaning of the letter should be flexible according to the place where it is written and not lose its meaning. He said that scholars have the disadvantages that they could’t grasp the intention quickly because they can not precisely analyze the writing. Toegye looked for the implicated meaning when he read the letter, found the unique one of the phrase, and did’t pass away the fine point of each word and alphabet. Toegye did’t erase or correct the letters of former scholars. He just added notes on the lines or his thoughts that any letter should be written in any letter. In his studying the sentences, We can see how he tried to meticulously, delicately, and precisely discern their meaning.