What would be correct? And why?
- I'm helping maintain the library.
- I'm helping to maintain the library.
- I'm helping maintaining the library.
- I'm helping in maintaining the library. (ok this one sounds quite wrong)
What would be correct? And why?
You use the verb 'help' with or without a 'to infinitive'. The meaning remains the same.
You don't help yourself but you help others. Your third sentence sounds like you're helping yourself in maintaining the library.
I'm helping _ in maintaining the library. You don't fill in that blank. The object of help could be anything but you.