When you do want to insert a <br /> break tag using Markdown, you end a line with two or more spaces, then type return. 当你想使用 Markdown 插入一个 <br /> 断行标签时，你需要在行尾输入两个或更多空格，然后按回车键。 Yes, this takes a tad more effort to create a <br />, but a simplistic “every line break is a <br />” rule wouldn’t work for Markdown. Markdown’s email-style blockquoting and multi-paragraph list items work best — and look better — when you format them with hard breaks. 是的，创建一个 <br /> 需要更多一点的努力，但简单的「每一行换行就是 <br />」规则并不适用于 Markdown。Markdown 的电子邮件风格块引用和多段落列表项，用硬换行格式化效果最好 —— 看起来也更美观。
哎呀，创始人早就写好了这个规则了啊。
GFM 中的规则
去找了一下 github flavored markdown，发现这里的说法是换行有软硬换行之分，
也就是说软换行就是
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<p>换行 换行</p>
而 HTML 中，多个连续的空白字符（包括换行、空格和制表符）都会被浏览器解析为一个空格。所以会被解析为：
1
换行 换行
硬换行就是
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<p>换行<br/> 换行</p>
这和原始的那个规则好像大差不差，总之就是，<br/> 的话就要用两次及以上的空格！
GitHub Flavored Markdown 上的中英对照
6.12 Hard line breaks A line break (not in a code span or HTML tag) that is preceded by two or more spaces and does not occur at the end of a block is parsed as a hard line break (rendered in HTML as a <br /> tag): 换行符（不在代码跨度或 HTML 标签中）若前面有两个或更多空格且未出现在块末尾，则会被解析为硬换行（在 HTML 中渲染为 <br /> 标签）：
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foo(空格)(空格) baz
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<p>foo<br /> baz</p>
For a more visible alternative, a backslash before the line ending may be used instead of two spaces: 若需更直观的替代方案，可在行尾使用反斜杠代替两个空格：
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foo\ baz
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<p>foo<br /> baz</p>
More than two spaces can be used: 可以使用超过两个空格：
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foo(空格)(空格)(空格)(空格)(空格) baz
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<p>foo<br /> baz</p>
Leading spaces at the beginning of the next line are ignored: 下一行开头的空格会被忽略：
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foo(空格)(空格) (空格)(空格)(空格)(空格)(空格)bar
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<p>foo<br /> bar</p>
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foo\ (空格)(空格)(空格)(空格)(空格)bar
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<p>foo<br /> bar</p>
Line breaks can occur inside emphasis, links, and other constructs that allow inline content: 换行符可出现在允许内联内容的强调、链接等结构中：
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*foo(空格)(空格) bar*
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<p><em>foo<br /> bar</em></p>
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*foo\ bar*
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<p><em>foo<br /> bar</em></p>
Line breaks do not occur inside code spans: 换行符不会出现在代码跨度内部：
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`code(空格)(空格) span`
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<p><code>code span</code></p>
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`code\ span`
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<p><code>code\ span</code></p>
or HTML tags: 或 HTML 标签中：
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<ahref="foo(空格)(空格) bar">
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<p><ahref="foo(空格)(空格) bar"></p>
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<ahref="foo\ bar">
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<p><ahref="foo\ bar"></p>
Hard line breaks are for separating inline content within a block. Neither syntax for hard line breaks works at the end of a paragraph or other block element: 硬换行用于在块内分隔内联内容。硬换行的语法在段落或其他块元素末尾无效：
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foo\
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<p>foo\</p>
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foo(空格)(空格)
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<p>foo</p>
1
### foo\
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<h3>foo\</h3>
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### foo(空格)(空格)
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<h3>foo</h3>
6.13 Soft line breaks A regular line break (not in a code span or HTML tag) that is not preceded by two or more spaces or a backslash is parsed as a softbreak. (A softbreak may be rendered in HTML either as a line ending or as a space. The result will be the same in browsers. In the examples here, a line ending will be used.) 普通换行符（不在代码跨度或 HTML 标签中）若未被两个以上空格或反斜杠修饰，则解析为软换行（在 HTML 中可能渲染为换行或空格。浏览器效果相同。以下示例使用换行符）：
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foo(无空格) baz
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<p>foo baz</p>
Spaces at the end of the line and beginning of the next line are removed: 行尾和下一行开头的空格会被移除：
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foo(空格) (空格)baz
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<p>foo baz</p>
A conforming parser may render a soft line break in HTML either as a line break or as a space. 合规解析器可将软换行在 HTML 中渲染为换行或空格。
A renderer may also provide an option to render soft line breaks as hard line breaks. 渲染器也可提供将软换行渲染为硬换行的选项。