While starting our writing process we have a great tenancy to copy and paste others' writing in ours. It is plagiarism even when we keep the reference. But of course we can use the definition or other passages of others in small quantity with reference. Otherwise we should digest and remodel the write up for any ideas we have taken from others. It should also contain reference as it is not our idea. But the sentences are ours as we created or remodeled it. This processing of sentences avoid plagiarism as what I understood. May be we have to be little slow in making our write ups. May be we have to concentrate more on quality than quantity. Its any way learning time to avoid plagiarism. Plagiarism tastes bad. Its writing time, learn to beat plagiarism.
Plagiarism is like dopping in sports