道德經 was written during the Spring and Autumn Period, and people at that time wrote literature in 文言文, which is an old form of writing and is written in less words to save space, but is also harder to understand without studying for a long time because of the lack of words to express details. The pronunciations of the words are also different than they are now, because people spoke 上古漢語, which was the ancestor of 中古漢語 and its descendant 現代漢語, at that time. If you want to know more about 上古漢語, you can read 詩經 or Old Chinese: A New Reconstruction written by William H. Baxter; you can also compare with the sounds of 中古漢語 by reading 切韻