1) In Laravel 5 , form helper is removed .You need to first install laravel collective .
Refer link: https://laravelcollective.com/docs/5.1/html
{!! Form::open(array('route' => 'log_in')) !!}
OR
{!! Form::open(array('route' => '/')) !!}
2) For laravel 4, form helper is already there
{{ Form::open(array('url' => '/')) }}