There are three ways to reply to an existing post on this forum:
1. Quote
When you press this button, the forum will automatically insert the text of the post as a quote:
You can then add your own text in reply.
Please don't just quote entire posts verbatim - it clogs up the thread with unnecessary repetition, and makes it hard to follow.
Instead, cut out all superfluous text - leaving just enough to give context; eg,
Note: If you select some text in the original post before pressing the 'Quote' button, the forum might insert only that text in your reply.
You can have multiple [ quote ] blocks within a reply - you can copy & paste the 'quote'...'/quote' tags.
Or use the 
button on the toolbar.
When posted, your reply will look something like this:
Note that the forum has automatically inserted a reference to the original post.
2. Reply
When you press this button, the forum will just give you a blank comment form:
Again, when you post, the forum will automatically inserted a reference to the original post:
3. Add New Comment
There is always an 'Add New Comment' form at the end of a thread (unless it's locked).
Using this will just put your comment at the end of the thread - without any context or reference.
You can, of course, manually insert quote blocks if you wish.