How to perform a line break? [duplicate]

I entered text with line breaks in the body but it doesn't show in the question.

marked as duplicate by José Carlos Santos, JonMark Perry, TheSimpliFire, mechanodroid, Mostafa AyazSep 2 '18 at 15:06

• Two line breaks is a paragraph break, two spaces at the end of the line cause a line break without a paragraph break. See also help/formatting. – Asaf Karagila Aug 26 '18 at 8:53
• See also: How to start new line on MSE? – Martin Sleziak Aug 26 '18 at 9:30
• I have noticed that you included (mathjax) tag - so you want to ask how to make a line break in the math mode? If you are asking about regular text (not formulas) then (markdown) is a more suitable tag and the links given in the previous comments should give you an answer to this. – Martin Sleziak Aug 26 '18 at 9:31

Input: Output:

Period. Period. Period and two spaces.
Period. Period. Period.

Period. Period.

Line break within math mode.
$$x + y = 2\\x - y = 4$$
Output:
$$x + y = 2\\x - y = 4$$