$('table tr:nth-child(3)').after('<div>....</div>');

每次都会在第三行之后插入内容！

This will insert something after the 3rd row every time!

$('table .one:nth-child(3)').after('<div>....</div>');

这将在第3个.one类之后插入一些内容.

This will insert something after the 3rd .one class.

$('table .two:nth-child(1)').before('<div>....</div>');

这将插入第一个.two类之前.意味着它将在.one&之间. .two.

This would insert before the first .two class. Meaning it would be inbetween .one & .two.

据我对您的评论的了解，您想在最后一个.one tr之后插入一个元素.

From what I understand of your comment, you would like to insert an element after the last .one tr.

可以使用以下方法轻松完成此操作:

This can easily be done by using this:

$('.one').last().after('<div>...code goes in here...</div>');

OR

$('.one:last-child').after('<div>...</div>');