How do I create a line border in Microsoft Word 2010?

Hi

To only add a line between paragraphs or the contents, you can insert it from the shapes and after that change its formatting like (weight, color, shadows, ...etc) from the format tab.

 

Previous answers are excellent for borders, but if you mean a line between areas in a page then either bottom only border as above or insert object and choose from the ruler designs

To add a border around text

 Home menu option > Paragraph group > Boarders > Outside boarder

 

for Table, click anywhere in table

Design menu > Boarders group > Boarder > Outside Boarders

 

Without knowing what exactly mean by line border, In MS Office Applications, you can create standard and custom borders from Home>Paragraph tab in Ribbon.