How do I add a span class to form item title in register form?
You can use the following code:

'#title' =&gt; '&lt;span class="myclass"&gt;' . t('I have read and agree to the &lt;b&gt;&lt;a href="@terms" target="_blank"&gt;terms of service&lt;/a&gt;&lt;/b&gt;', array('@terms' =&gt; url('terms'))) . '&lt;/span&gt;',