Here is an example of acceptable and unacceptable paraphrases:

Original Version The craft of hurricane forecasting advanced rapidly in the sixties and early seventies, thanks to fast computers and new atmospheric modeling techniques. Now there is a lull in the progress, strangely parallel to the lull in the storm cycle. The National Hurricane Warning Center shoots for a 24-hour warning period, with 12 daylight hours for evacuation. At that remove, it can usually predict landfall within 100 miles either way. Longer lead times mean much larger landfall error, and that is counterproductive. He who misses his predictions cries wolf. (From "Our Barrier Islands," by William H. MacLeish, Smithsonian, Sept. 1980, p. 54.)

Unacceptable Paraphrase (Plagiarized sections are in bold type.) Hurricane forecasting made rapid progress in the 60s and 70s due to fast computers and new atmospheric techniques, but there is now a lull in the progress. The Warning Center tries for a 24-hour warning period, including 12 hours of daylight. That close to the storm's arrival, the Warning Center can usually predict landfall within 100 miles either way. If lead times are longer, there will be a much larger error, which will be counter-productive (MacLeish 54).

Original Version The craft of hurricane forecasting advanced rapidly in the sixties and early seventies, thanks to fast computers and new atmospheric modeling techniques. Now there is a lull in the progress, strangely parallel to the lull in the storm cycle. The National Hurricane Warning Center shoots for a 24-hour warning period, with 12 daylight hours for evacuation. At that remove, it can usually predict landfall within 100 miles either way. Longer lead times mean much larger landfall error, and that is counterproductive. He who misses his predictions cries wolf. (From "Our Barrier Islands," by William H. MacLeish, Smithsonian, Sept. 1980, p. 54.)

Acceptable Paraphrase During the past thirty years, powerful computers and new techniques which allow modeling of the atmosphere have significantly increased the accuracy of hurricane forecasting, though there have been no improvements in forecasting during the past few years. However, now it is possible to predict where a hurricane will hit land with an error of not more than 100 miles if a warning of 24 hours is allowed. If more than 24 hours is required, the error will be greater. Repeated forecasting errors will cause the public to ignore the warnings (MacLeish 54).

Taken from: Avoiding Plagiarism at the University of the Sciences in Philadelphia website: http://www.usciences.edu/writing/plagrsm.shtml (last visited 10/2/12)