Slim templates on Ruby on Rails, what are best practices

For new line, you should just use br like:

h1 Line1 content
br
h1 Line2 content

And about the above mentioned code, it can be rewrite like this:

-provide(:title,@course.title)                                                    
.row
  aside.span4
    section
      h1 = @course.title.capitalize

      = link_to t('ui.edit'), edit_course_path(@course)
      '|
      = link_to t('ui.back'), courses_path

      p
        b = t('activerecord.attributes.subject.title')
        |:     
        = @course.subject.title

      p
        b = t('activerecord.attributes.student_level.title')
        |: 
        = @course.student_level.title

      h4 = t('activerecord.attributes.course.objectives')
      = @course.objectives

To insert br tag into some tag in slim:

Example 1. Slim template:

h1
  | Hello
  br
  | world

It will produce html:

<h1>Hello<br>world</h1>

Example 2. Fragment of slim template for displaying form:

p
  = f.label :title
  br
  = f.text_field :title

It will produce html:

<p>
  <label for="question_title">Title</label><br>
  <input name="question[title]" id="question_title" type="text">
</p>