How to build a sentence based on tags
I think it'll be easier and more efficient to achieve that if you use the loop functionality instead of querying the actual tags. I think this code does what you want:

{% set topics = '' %}

{% for topic in entry.topics %}
    {% set topics = topics ~ topic ~ (loop.index &lt; loop.length-1 ? ', ' : loop.index &lt; loop.length ? ' and ') %}
{% endfor %}

&lt;p&gt;This is a sentence with {{ topics }}.&lt;/p&gt;