In the course of the Renaissance, as vernacular languages became more important in relation to Latin, it became increasingly necessary to establish rules for their use. This led to the production of grammars, the first of which was Antonio Nebrija's Castilian Spanish grammar in 1495. For us the question arises as to when the first Dutch grammar was produced. Well, here there are in some sense two answers. One answer is that the first Dutch grammar was Twespraeck der Nederduitsche Letterkunst, first published in 1584. It is thought that the probable author of this was the Amsterdam rhetorician Hendrick Laurenszoon Spiegel. It is a fairly extensive grammar containing many of the features we have come to associate with such books.
However, another answer is an anonymous manuscript dated 1568, i.e. 18 years earlier than Twespraeck, discovered in a country house in Zeeland in 1975. This has the title Voorreden vanden Noordich ende Nutticheit der Nederduytscher Taelkunste ('Argument for the Necessity and Usefulness of Dutch Grammar'). After much detailed study, Karel Bostoen came to the conclusion that the manuscript was the work of Johannes Radermacher, a German-born merchant who was living in London in 1568. Could it be that the first Dutch grammar was written not by a Dutchman in the Low Countries, but by the Aachen-born Radermacher in England? Yes and no. There are problems with this grammar. The most obvious is that it is unfinished, dealing only partially with one of the three themes set out in its introduction. Furthermore, it was not published, but remained in manuscript, and therefore is likely to have had little influence on subsequent Dutch grammars. On the other hand, it does provide an analysis of types of word, such as noun (namelijk) and verb (wervich) and discusses the sounds of the Dutch language. Perhaps, somewhat provocatively, the answer to the question depends on one's perspective. Some Dutch scholars are inclined to name Spiegel's work as the first Dutch grammar, while others such as the present author may prefer to name Radermacher as the author of the first Dutch grammar.
GRW Dibbets (ed.). Twespraack der Nederduitsche Letterkunst. Assen, 1985.
Christopher Joby. The Dutch Language in Britain (1550-1702). Leiden, 2015, pp. 323-325.