Is it OK to put my input on articles I found from the past? Will not many people be up to respond due to the date? When people stop posting for years, does this mean the thread has "ended" ?

I think you should post your thoughts. Many of us weren't hanging around Ballet Talk years ago and could find some of those topics extremely interesting. Certainly I've seen older topics resuscitated, with animated discussion following as a result.

As long as the topic isn't tucked away in archives, yes. If it is an archived thread, feel free to open a new one.

The membership of this board is somewhat fluid. We're always welcoming new members, and some haven't posted in years. It's always interesting when someone adds a post to a thread which has been dormant for a while. Sometimes it sparks a whole new discussion.

Just to be sure, all I have to do is reply - not click something else or open something new? Just continue where it was left off? I don't even know what those archive articles are.

The Archives -- mostly topics that were timely more than three years ago -- are listed at the bottom quarter (approximately) of the Home Page.

Seems to me you've figured out the buttons very well. You opened a new topic (here) and replied to another -- both with apparent ease. You might want to read further in "How This Site Works". I think most questions are addressed there.