Is it plagiarism to contribute with famous poems?

Generally, more than 30% similarity is plagiarism, and more than half is serious plagiarism. But words are easy to judge, and sometimes it is really difficult to judge whether to use creativity or draw lessons from plots, paragraphs or even characterization.

But some normal readers can still see it. Most importantly, it's not bad if all you borrow is to explain your point of view and writing purpose. If you just copy it, and the things you express are as original as others, it is really plagiarism.

Just like we use celebrity stories in our compositions, you use them and I use them. This is not plagiarism, but if you use them to illustrate your point of view, I also use them to illustrate the same point of view as you. That's plagiarism. I'm not creative at all. Poetry is the same, it is better to indicate the source.