Layout of a simple PHP form needs altering

Question

I'm trying to create a simple inline form that displays an entry field followed by a submit button - how can I alter this existing code so that these elements display horizontally in one row? ...I've tried altering the form class but no success. Thank you in advance.

PHP

<div class="d3-mailchimp" data-block-id="<?php echo $bID; ?>">
<?php 
if (isset($errors)) {
    $errors->output();
}

if (isset($message)) {
    echo '<p class="message">'.$message.'</p>';
} else {
    ?>
    <form method="post" action="<?php echo $this->action('submit') ?>" onsubmit="d3_mailchimp_submit(this); return false;">
        <?php $token->output('d3_mailchimp.subscribe'); ?>

        <div class="form-group field">
            <?php    
            echo $form->email('email_address', '', [
                'required' => 'required',
                'placeholder' => t(''),
            ]);
            ?>
        </div>

        <?php
        if ($showTermsCheckbox) {
            ?>
            <div class="accept-terms">
                <label for="accept_terms">
                    <?php
                    echo $form->checkbox('accept_terms', 1, 0, [
                        'required' => 'required',
                    ]);
                    ?>
                    <?php echo $acceptTermsText; ?>
                </label>
            </div>
            <?php
        }
        ?>

        <div class="submit-button">
            <?php 
            echo $form->submit('submit', t('Subscribe'), [
                'class' => 'button',
            ]);
            ?>
        </div>
    </form>
    <?php    
}
?>