How do I add a line in between 2 lines of text?

Does anything so far solve your issue, @Eric5CCA ? If not, draw a line with the line tool so we can see what you are trying to achieve and show a screen shot, then we'll tell you how to do it with underlines, ruling lines, or borders so you can use a paragraph style.

~ Jane

Two options to add a line under a paragraph—underlines and paragraph rules. Adjust the offset value to move the lines up or down the page.:

In both cases, set the paragraph spacing as desired. Then select the top paragraph:

1. Click the Underline button to assign the underline. Alt/Opt click the Underline button and adjust the weight and offset. Note: Underlines are always the width of the text. 
   
2. Enable Rule Below paragraph in Paragraph rules. Adjust the weight and offset. Note: para rules also allow you to define the rule as text-wide, frame-wide or use custom left and/or right indents for optimal control.
   

Both can be saved as part of a style and assigned with a single click. 

~Barb 

Do you want a line to separate 2 paragraphs or under each line of a paragraph?

For the first solution, you can use a rule: see in the paragraph option “rule below” or “rule above”. For the second solution, you can use the underline function.