At the bottom of each post is a “comment” button. Click on this, and you will see what others have said in response to the post. Scroll down, and there is a place to add your own comment. I like to think of the comments following a post as a conversation. But these are conversations which are freed from the constraints of real time. There is no need to jump in and say something, just to keep the conversation going. There is time to be thoughtful, and even to research before you write.
The first rule of a good comment is to respond to what has come before you. The comment should pertain directly to the original post, and/or to one of the other comments. I don’t mind digressions, as long as they are interesting digressions.
The second rule is to say something that adds value to the conversation. You may have a different point of view. Share it. But try to articulate why your view is different, and what you are basing your views on.
The third rule is to give proper attribution for your sources of information. For example, if you say that research has shown something, tell us where you learned about that research. If you can look up the original research and provide us with that source, that is even better.
I don’t think there are any other rules, just suggestions:
Keep it short. A comment in the neighborhood of 100-200 words is probably just right. That is enough to make a point, and support that point with a little background information, and to cite your sources. If you go over 300 words, you should think about a post instead. If you have more than one or two sources to cite, think about either writing a post, or writing two comments.
Keep it focused. Think about what the main thing you want to say is, and say it. A comment that tries to make three or four points about the original post just ends up being unfocused and boring.
Be yourself. The most interesting comments to read are the ones that provide a personal opinion and real life experience.
Add to the information. Not every comment will benefit from having a link or reference. But when you can support what you want to say with information from another source, you should do it. And remember, you have an obligation to give credit to the original source for ideas that are not your own. See the document on citations and references.