Layout of a simple PHP form needs altering - php

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
}
?>