After login, go the course where you have your discussion forum activity.

Click on the forum activity.

You will be taken to the forum activity page, where all the forums will be listed.  Select the particular forum you want to reply to.

Click on the reply button available at the bottom right corner of the post.

After clicking on the reply button, a box will appear in which you can post your reply to a forum. Then click on "Post to forum”.

Your reply is now posted 

As a user, you can “Permalink” your post, which means it allows you to link directly to a specific forum post so you can share it easily with others. You will see the permalink option when clicking to reply to a post. 

You can also edit your forum post even after posting the reply by clicking on "Edit."

You can also delete the post by clicking on “Delete”.