He was involved in the development of the Riksmål and Bokmål form of the written Norwegian language. He was also one of the first to propose Norwegianization, the rewriting of loanwords into a Norwegian spelling (e.g. turning chauffeur into sjåfør). His aim was to give a more Norwegian coloring to the literary language of Norway by adapting the orthography and syntax to Norwegian usage, and by substituting wherever it was possible Norwegian words for foreign derivatives. The most comprehensive treatment of the subject may be found in his Unorsk og norsk, eller Fremmedords avlösning (1879-1881).
While Knudsen was extreme in his views and frequently erred through a lack of thorough philological training, he exercised a decisive influence upon his contemporaries, especially Björnstjerne Björnson.