With the bridge scenario there are no absolutes. Since you are shooting 35mm you have the luxury that large format shooters can ill afford - you can burn shots experimenting. Try it a number of ways: bridge level, horizon level, as many different angles as you have access to. Keep notes on what you're doing, if it will help you. Then compare what you did with the results on the film - this is part of the learning process, and then the next time you are looking through the viewfinder at a bridge you will feel a lot more confident about how you want to compose the shot. It may come down to deciding to use the bridge to mask the actual horizon.
As far as foregrounds and backgrounds, first you decide where your main subject resides. If it is in the background, then the general rule of thumb is that whatever is in the foreground should lead the viewer's eyes to the main subject. If the main subject is in the foreground you must first decide how much background you want to show. The main problem in my own compositions like this is that I never get close enough and end up with too much background - when in doubt move in closer. Something to become aware of is that our eyes are amazing machines that make all kinds of adjustments to what we end up thinking we are seeing, while cameras and film are much more limited and just basically record what's in front of them. That sometimes presents huge discrepancies between what you thought you were photographing and what you get on the final print. That's why Ansel Adams and everybody down through our own Les McLean talk about Pre-visualization. A simple definition of that term is "learning to see the way your camera does. That is accomplished by going through lots of film. Yes, it helps to read the books and attend the workshops and take classes, but in the end you do all your real learning about composition with the camera in your hand. But on the other hand it is good and necessary in the beginning to observe the various helpful hints like the Rule of Thirds - you have to know what the rules are before you can throw them out.
The only other idea about composition that will be really useful to you right now, and is related to the Rule of Thirds, is avoid the "bullseye" approach - putting the subject dead-center in the frame. That is often the first step from taking snapshots to making photographs. I hope some of this is useful to you.