LibreOffice

Writer

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

Pages

• You can edit the page style at the bottom of the application screen (left click to change the page style).
• If the footer is different on left and right pages, edit the page style, Footer tab, and check the checkbox "Same Content Left / Right".

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.

Calc

Merged cells

• You can see if some cells are merged by clicking on a row / column head. Then all cells belonging to the row are highlighted in blue except those that are merged.
• To unmerge: Format -> Merge cells.