Hey Jacqueline! If I understand your question, I think I can help. So if you look at the examples you gave, the word effort is preceded by "an" which implies the subject is singular. Efforts would be used in phrases such as "Her efforts really paid off!" If you get rid of "an," then you're no longer implying that there was only one effort being made. So using the example I gave, maybe the girl you're talking about put effort into taking notes in class, studying, doing homework, etc. This is when you would imply multiple efforts. I'm not sure if that makes any sense, and I hope I didn't confuse you more haha it's a little difficult to explain.

Did you do a web search yet? If I do a Google search for "efforts" (in quotes for an exact match), then both the 4th and 5th results actually have an example of the word in a sentence. Those are pretty good results. I don't know whether that'll work from your locale, but hopefully it does.