Hola Manuel,
In reviewing the Word document you provided and going through your posting. I would suggest for you to double click the line of interest, this will reflect the certices. At the location where you want to split your line, you want to hover over the line or vertice, and then use the Split Line tool. This will split the line at that location, and then you can delete the line not needed. Please let us know if you have any further issues. Gracias, Manuel!