Plagiarism

Plagiarism is the act of using exact or similar ideas, words or sentences from another author as one's own work without proper citations. If one hands in the same piece of work for two classes, it is also regarded as plagiarism.
Before I begin my studies here, I think plagiarism only refers to copying/stealing from others' work and inappropriate paraphrasing of the same idea. I did not know that handing in the same piece of work, which is geniunely written by me, for two different classes is also considered as plagiarism. I did not know that paraphrased sentences need to be cited either.
I have learnt about plagiarism in one of my classes last semester when we were going to write our research papers. Since we had to describe 3 experiments from scholarly journals, we had to keep an eye on unintended plagiarism. Sometimes it is hard to rephrase sentences from a journal article as I usually regard the way which the source explains the issue is the best. Due to this, I always check the already-paraphrased sentence twice before moving on to the next point, in order to make sure that I did not unintentionally copy anything while keeping its meaning.