Unintentional plagiarism is a serious matter in graduate school. Unintentional plagiarism most often occurs when a student loses his or her own voice. That is, instead of the student remaining in control of what he or she is writing, the student begins to rely on "experts" to say what the student should be saying with her or his own voice. The scenario goes something like this: the student finds a source with whom he or she agrees and, making a few changes in wording, basically follows the structure of the source material. Even if citations are used, this is a recipe for plagiarism. The following are some commonsense suggestions to help you avoid unintentional plagiarism.
- Use your own voice. Put what you have to say in your own words. Don't hide behind the "experts."
- Never compose essays with source material open. Take notes from source material on note cards and compose from those cards.
- Clearly distinguish summaries from quotations.
- Keep quotations brief and few. Use direct quotes as spices in a meal, not as the main dish.
- Use block quotations only when you plan to give extensive analysis of the quoted material.
- Always introduce quotations (e.g. According to Helmer, "….").
- Summarize or paraphrase material using sentence structure that differs significantly from the source. (Changing a few words, but keeping the same basic sentence structure of the original is still plagiarism.)
- Provide a citation for all quoted, paraphrased, or summarized material. When in doubt, provide a citation.
Original Source #1
Cheap grace means the justification of sin without the justification of the sinner. Grace alone does everything, they say, and so everything can remain as it was before. 
Plagiarized Version #1A
Cheap grace means the justification of sin but not the justification of the person who sinned. Some people say that grace alone does everything, so everything can remain the same. 
Even though the writer has cited the source, quotation marks were not used around direct quotations such as "cheap grace means the justification of sin" and "grace alone does everything."
Plagiarized Version #1B
Cheap grace is taking care of sin without dealing with the one who sins. God's unmerited favor alone takes care of everything, some say; so the situation can stay as it was before. 
Still plagiarism. Replacing key words with synonyms but keeping the basic sentence structure of the original is still plagiarism, even if you provide a citation.
Acceptable Version #1C
According to Bonhoeffer, cheap grace refers to the theological position that a sinner can be justified before God without changing his or her behavior. According to this theological position, one does not need to change to receive justification for it is provided by grace alone. 
This is a legitimate paraphrase of Bonhoffer's words. Note that it is both introduced and footnoted. Also note that short phrases (usually three words or less) do not necessarily need to be placed in quotation marks, especially if the sentence structure is completely different from the source.
Original Source #2
As Christianity spread, and the Church became more secularized, this realization of the costliness of grace gradually faded. The world was Christianized, and grace became its common property. It was to be had at low cost. Yet the Church of Rome did not altogether lose its earlier vision. It is highly significant that the Church was astute enough to find room for the monastic movement, and to prevent it from lapsing into schism. 
Plagiarized Version #2A
Christianity spread throughout the Empire but at the same time it became more worldly; thus the realization of the costliness of grace gradually faded away. As the Empire embraced Christianity, grace became its common property and was available cheaply. Nonetheless, the Church of Rome did not completely lose its earlier vision. That the Church was smart enough to make room for the monastic movement and to keep it from dividing from the Church is highly significant.
Plagiarism: not only does the writer copy the outline and sentence structure of the original, she or he fails to place within quotation marks long phrases taken directly from the source such as "the realization of the costliness of grace gradually faded" and "grace became its common property." Moreover, there is no citation.
Plagiarized Version #2B
According to Bonhoeffer, Christianity spread throughout the Empire but at the same it became more worldly; thus "the realization of the costliness of grace gradually faded." As the Empire embraced Christianity, grace became its common property. Nonetheless, the Church of Rome did not completely lose its earlier vision. It is highly significant that the Church was "astute enough to find room for the monastic movement," and to prevent if from "lapsing into schism." 
Still plagiarism. Although the writer has introduced the material and included a citation and placed some quoted material in quotation marks, other direct quotations are not in quotation marks such as "grace became its common property." Moreover, the sentence beginning with "nonetheless" is a direct quote with only two words replaced by synonyms. Finally, the overall structure of the paragraph and of each sentence mimics the original too closely.
Acceptable Version #2C
Bonhoeffer argues that monasticism within the Church is evidence that the concept of costly grace was not completely lost after Christianity became the state religion. Nonetheless, the Church was largely secularized during this period; and for most, grace "was to be had at low cost." (Bonhoeffer, Discipleship, 49).
Notice that the paraphrased information is introduced and a citation is provided; both the over-all structure and the sentence structure differ significantly from the source. One small quotation is used to add flavor.