woocommerce_before_checkout_form action-hook . WC 1.0

add_action( 'woocommerce_before_checkout_form', 'action_function_name_2383' );
function action_function_name_2383( $checkout ){
	// action...
}

In file: /templates/checkout/form-checkout.php

woocommerce_before_checkout_form

do_action( 'woocommerce_before_checkout_form', $checkout );

add_action( 'woocommerce_before_checkout_form', 'woocommerce_checkout_login_form', 10 );

add_action( 'woocommerce_before_checkout_form', 'woocommerce_checkout_coupon_form', 10 );

add_action( 'woocommerce_before_checkout_form', 'woocommerce_output_all_notices', 10 );