The crumbling bridge must be repaired.
Does the present participle "crumbling" here mean that the bridge is crumbling?
I think that "crumbling" is not a progressive action because it happen quickly. Therefore, I think it is better to use the past participle "crumbled". Is this right?
The crumbled bridge must be repaired.
Here, "crumbling" is better. A bridge or any other structure can go on crumbling for years, my own house is an excellent example. It means that a lot of small pieces (like the crumbs of a cake) keep falling off the structure. After the bridge, or my house, has been crumbling for years it might quickly collapse.
not a teacher