Most permissions in the Forums portlet are specific to a single instance of the portlet. So you might have broad permissions in one portlet instance but not another.
Some permissions that may vary among different Forums portlet instances include the following:
Your ability to publish posts.
Your ability to publish replies to posts.
Your ability to edit and delete your own posts and replies.
In general, you can tell whether you have permission to do something if the appropriate buttons are displayed (for example, if you see the Add a Post button, you have permission to post). If you are unable to do something you think you should have permission to, see an administrator for the page or context that hosts the Forums portlet instance.
Note also that your ability to post and reply might be restricted in the following situations:
Sometimes posts may be set up to allow only one reply per user -- so if you have already responded once, the Reply button will not display again.
If a topic has been set up to be read-only, the Add a Post and Reply buttons will not display in that topic, regardless of of what permissions you have. You can tell whether a topic is read-only if it is marked with an eye icon.