The writer needs to take the research process a step farther. If the instructor is lenient, you may be asked to rewrite the paper. Perhaps the most difficult part of the research paper process for most students is paraphrasing sources. In order to paraphrase a passage from a source, you have to understand it very well. You have to know what all the words mean, at least in that particular context, and you have to know other words that have similar meanings.
Some students copy the passage and then try to substitute new words in the same sentence structure.
The result has the same grammatical structure as the original, with some of the words changed. Others will keep the same words, but reorganize the sentence structure, perhaps re-ordering the sentences at the same time. Neither of these approaches, same structure but different words, or same words but different structure, is sufficient to avoid plagiarism, but each is a step in the right direction. If you are trying to make these sorts of transformations, and you document your sources, it is unlikely that your instructor will accuse you of plagiarism, although he or she may suggest that you are too dependent on your sources for language and sentence structure. If you are doing these things, you are trying, and you are learning.