Fore/back ground can be kept in focus if it relates to the subject and helps lead the eye. (converging/diverging lines) Keep it out of focus if it is "clutter" (distracting). Sometimes the background just out of focus so not be distracting, but in focus enough to tell what it is is a nice effect and helps viewer to relate (Kid on a swing with slightly out of focus kids playing in the backgroud)
Fill the frame (viewfinder) and you'll have less to crop in the darkroom. Don't try to include everything your eye sees. Just doesn't work.(for me anyway).
Bushes, trees, fences, lightpoles can often be used to "frame" the subject. Effective for land/cityscapes.
Most importantly -for every rule, there is an exception- except this one... ;)
Theories of composition are quite varied. If you show your pictures to people you want them to experience them. The whole picture, not just part. There are general rules and they are well known, but they too can work against a photograph if they are not used in a way that compliments each other. A curved road should not lead away from the subject as it draws the eye away and so on and so forth
If it were me:
I would place the bridge above or below the horizon makeing it echo the horizon as a repeating line.
Bushes are great to have in the forground as they can be interesting. Unfortunately they can also clutter the view forcing the viewer to look only at the bush. Personally I thik this means making it very dominant and huge. Use the branches to frame with or point to the repeating lines of the horizon and the bridgethis should lead the viewer around the photograph.
My two cents
I think it's a bad idea to learn rules about composition. It's better to learn what effect elements of design have on you and how they relate to what you want to see. Level bridges crossing the format are painfully calm to me but then maybe sometimes I want to make a painfully calm picture. Quit composing subjects and start looking at forms and values and most of the rules worthy of mention will automatically be followed.