LibreOffice

Writer

• To create a line break, use SHIFT + ENTER. This won't create a new paragraph.

Paragraphs

• If there is a thin black line appearing somewhere, it is most likely a paragraph border. To remove it, edit the paragraph formatting.

Numbering and Bullets

• When creating a new paragraph (typing ENTER), a new number / bullet is automatically inserted. You can remove it by pressing BACKSPACE.
• I don't know how to have two or more levels of numbering / bullets on the same line.