The following tutorial explains how to create paragraphs and line breaks in WordPress.
To add a new paragraph in the content editor, you simply need to press the <Enter> key. WordPress will add a new paragraph with double line spacing.
However if you only want a single line break, then you need to press <Shift><Enter> together.
This will tell WordPress that you want to start a new line inside an existing paragraph thus it needs single line spacing vs double line spacing.
If you want to remove a paragraph break or single line break, then hit <Backspace> for double line and <Shift><Backspace> for single line.