You can override the automatic formatting determined by the styleStyle.tex of your document to add horizontal spaceHorizontal.tex in text and mathematics, to add vertical spaceVertical.tex between paragraphs, or to create a new lineLine_breaks.tex or new pagePage_breaks.tex.
On the screen, the program breaks each line of text wherever necessary to fit the material inside the document window. If you change the size of the window, the program automatically adjusts the on-screen line breaks. The size of the printed page and the size of the document window are not the same, so the line breaks you see on the screen are not necessarily those that will appear in print. To examine the typeset spacing, preview your document.
You set the defaults for using the spacebar and the Enter key from the Edit tab sheetEdit_defaults.tex in the User SetupUser_setup.tex dialog box.