Drupal - How to check a checkbox by default in forms api
See below example...
$options = array();
$options["1"] = "One";
$options["2"] = "Two";
$options["3"] = "Three";
$options["4"] = "Four";
$form['ios'] = array(
'#title' => t(''),
'#type' => 'checkboxes',
'#description' => t(''),
'#options' => $options,
'#default_value' => array("1", "2", "3")
);
You Have Single Field like News letter or terms condition check box , You can use below code
$form['name']['terms_condition'] = array(
'#type' =>'checkbox',
'#title'=>t('Terms and conditions'),
'#required'=>TRUE,
'#default_value' =>TRUE, // for default checked and false is not checked
);