If you click on the "watch" tab at the top of a topic, or check the box in the quick reply box if you comment, the topic will appear in your "Watched Topics." This is at the top of the page :D:D:D
Then when you click on Watched Topics, a list of topics will appear that you are watching.
At the far right to the name of the topic is this symbol => click on it, and it will take you to where you left off the last time you were there. :D:D:D
This will also work anywhere on the board, for any topic that you have already opened and read.