Academic paraphrasing means more than just going through the original writing and using a thesaurus to swap each word in turn for a synonym. This will not provide you with something that really means the same as the original nor will it read well, yet this is what some students try to do when they paraphrase. To do academic paraphrasing well you have to:
Where most students err in terms of unintentional plagiarism is not in failing to acknowledge ideas, but in paraphrasing incorrectly. Many novice researchers assume that if paraphrasing means putting a passage "in your own words," this can be accomplished by simply substituting synonyms for key terms. Stealing syntax, or sentence structure, is as significant an offence as stealing an idea because, as you know from your own writing experience, trying to phrase a point exactly is a difficult art. Proper paraphrasing depends on thorough comprehension of material, not on a thesaurus.
Purdue OWL: Quoting, Paraphrasing, and Summarizing
To paraphrase means to restate someone else’s ideas in your own language at roughly the same level of detail. To summarize means to reduce the most essential points of someone else’s work into a shorter form. Along with quotation, paraphrase and summary provide the main tools for integrating your sources into your papers. When choosing which to use, consider first your discipline and the type of writing in which you are engaged. For example, literature reviews in science reports rely almost exclusively on summary. Argumentative essays, by contrast, rely on all three tools.