There are situations where you might want to deviate from the strict layout groups prescribed. A good example is, if you want the name field of a record to be rendered as a heading, the field's label will interfere with the appearance. In this case, don't put your field into a group element, but for example, into the plain HTML h1 element. Then, before the h1 element, put a label element with the for attribute set to your field name:

<label for="name" /> 
<h1><field name="name" /></h1> 

This will be rendered with the field's content as a big heading, but the field's name only in small above. That's basically what the standard partner form does.

If you need a line break within a group, use the newline element. It's always empty:

<newline /> 

Another useful element is footer. When you open a form as a popup, this is the good place to place the action buttons. It will be rendered as a separate bar too, analogous to the header element.