For a WordPress beginner (at least for me, who used to write html in notepad++), when editing a post, it is irritating that the 'Enter' key is not simply adding a line break but starting automatically a new paragraph instead, especially when typing some pieces of code.
Fortunately, there is a trick to add a line break without starting a new paragraph.
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A new paragraph is usually started after a "Hard break" also known as a "Carriage Return", which is indeed the default when typing "Enter" in a Word Processor. At the opposite, a simple "Line break" also known as "Soft Break" does not start a new paragraph and result in a normal line-spacing.
Ex.: The white-space between the previous text and this line is due to a "paragraph separator". But here after, you will see two lines A. and B. separated by a simple line break.
A. first line of text
B. second line of text
Instead of typing "Enter" at the end of the line A., press "Shift-Enter" instead. That wilt result in a "soft break" (line break) instead of a "hard break" (carriage return).