Can't access data in Matrix field
You're close! The relatedContent field is an ElementCriteriaModel, so you need to loop over it (or otherwise treat it somewhat like an array) to get to the entries.

The matrix field is also an array like thing, and you need to loop over it to get at the blocks. See here

Here is what I would do (assuming the matrix field handle is genericContent:

{% for oneRelatedContent in entry.relatedContent %}
  {% for block in oneRelatedContent.genericContent %}
    &lt;article class="t{% if loop.first %} t--tall{% endif %}"&gt;
      &lt;div class="content"&gt;
        &lt;span class="subtitle"&gt;{{ oneRelatedContent.sectionTag.first }}&lt;/span&gt;
        &lt;h1&gt;{{ oneRelatedContent.title }}&lt;/h1&gt;
        {{ copy }}
      &lt;/div&gt;
    &lt;/article&gt;
  {% endfor %}
{% endfor %}


You want to use loop.index to tell you which matrix block this is.
If you need to count the entries in relatedContent, you will need loop.parent.loop.index Loop variable