Hi,
You have to use a form of keyword arguments to provide parameters in {% url %} tag.
For example,
{% url post_view post_id=post_id %}
the former post_id is the argument name in your view function, the latter post_id is the variable name in your template context.
If you have more than one parameters, just append them to the list, separated by a space:
{% url post_view post_id=post_id tag='post tag' %}