_nav.html.erb

<div class="nav">
  <%= link_if_action ["new","edit","show"], t("project.back"), projects_path %>
  <%= link_if_action "index", t("project.new"), new_project_path %>
</div>

layout_helper.rb

def link_if_action(actions, label, path)
  if actions.include?(params[:action])
    link_to label, path
  end
end