It sounds like you're trying to combine a line made by typing underscore
characters (_) with the underline feature under some text. That never lines
up.
There are several ways to make your lines straight, and none of them involve
underscore characters. If you use underlined spaces or tabs, the underlined
text will create a continuous line, but that gets real awkward if you need to
add text before the line. The way I prefer is to insert a table that is
formatted so that only the bottom border of the cell is shown. Then you can
type whatever you want in the cell and align it however you want. I usually
do signature/date lines this way.
If your lines extend from one margin to the other, you can use a paragraph
border (Format - Borders and Shading). Apply a bottom margin to the
paragraph, and then type whatever you want.
There are probably other ways to type on a line, but these are the easiest.
Fred