NOT ADHERING TO A SPECIFIC REFERENCE STYLE (E.G. APA, HARVARD ETC.)?
Choice of reference style or indeed not using a formal reference style at all can never be the source of contention whether something is plagiarism or not. If used consistently throughout a paper and if it includes all the necessary information for a reader to be able to identify the exact source, reference style is immaterial.
However, your choice of style can go against formal requirements as stipulated by the individual school or university. In this case, your paper may be seen as not working within accepted bounds and will be sanctioned in accordance with the rules and regulations on academic conduct.
For this reason, it is always a good idea to check if restrictions apply and to ask your supervisor for advice.
The great advantage of using an established rather than a homemade style is that it is much easier to stay consistent, and indeed, if you are using a reference management software like Mendeley, only formally accepted styles will be available which means that you do not really have the choice.
SUBMITTING SOMEBODY ELSE´S PAPER AS YOUR OWN?
This is outright plagiarism. If you want to use elements from somebody else´s work, you need to make sure that the reader understands without equivocation what is yours and what is not. This is achieved by leaving a trail in the paper in the shape of an in-text citation and a corresponding reference in the bibliography.
If you lift a string of text verbatim (word for word) from a source, this constitutes a quotation and you need to surround the text with quotation marks followed by a parenthetical citation, e.g. (author, year).
If you are rendering somebody else´s ideas and thoughts in your own words (paraphrase), you need to insert a narrative citation immediately after the paraphrase, e.g. author (year) …
In both cases a reference should be included in the bibliography.
It goes without saying, that submitting an entire paper borrowed from somebody else as your own can never be warranted and is a serious breach of academic (and indeed any other) good practice.
RE-PHRASING SOMEBODY ELSE´S WORDS WITHOUT LEAVING AN IN-TEXT CITATION OR REFERENCE?
This is plagiarism and indeed a frequently occuring kind of plagiarism.
If you are paraphrasing somebody else´s work to make it better fit the overall style and narrative of your own paper, that is perfectly okay, as long as you make sure that the reader knows exactly what particular parts were included in this way. To accomplish this sort of transparency you need to insert a (narrative) citation and a reference as in:
According to author (year), correct reference practice is of the essence in academia.
Re-phrasing something could also entail deleting parts of a sentence and at the same quoting other parts, as in:
Solar, wind, and water power are all green and sustainable forms of energy (original)
“Solar […] power [is a] green and sustainable [form] of energy” (author, year, page)
In this case, you need to flag up all changes made and of course surround the whole thing with quotation marks to signal that it is a quote.
And remember, however way you decide to paraphrase, don’t forget to stay loyal to the meaning and intention of the original source; changing and deleting just a single word can have far-reaching implications
USING COMMON KNOWLEDGE / TRUISMS WITHOUT POINTING TO THE SOURCE?
This is not necessarily plagiarism.
“The world is round” is a universal and hardly controversial understanding shared by most (intelligent) people and leaving a citation and reference in this case could come across as redundant and indeed pedantic.
The same could be true for certain domain-specific concepts that have been around long enough to enjoy universal acceptance throughout the specific community.
These latter cases are, perhaps and probably, less obvious than absolute truisms like “the world is round”, so if you are not sure of the status of a concept, play it safe and leave a citation and a reference.
BUYING A PAPER ONLINE AND SUBMITTING IT AS YOUR OWN?
Purchasing a paper and submitting is as your own is not only a blatant case of plagiarism but also represents a complete absence of understanding for what it means to be a part of an academic setting.
CITING AND REFERENCING SOURCES BUT LEAVING OUT IMPORTANT INFORMATION (E.G. YEAR)?
More than anything, observing a referencing regime but forgetting key elements in the process is sloppy practice and not necessarily plagiarism. However, depending on the reader, i.e. the assessor, carelessness could easily be construed as plagiarism, so it does pay off to pay attention to the details and if you are doing the job anyway, you might as well do it properly and be safe.
ADOPTING AND PRESENTING SOMEBODY ELSE´S IDEAS IN YOUR OWN WORDS WITHOUT A CREDIT?
This is plagiarism, cf. Re-phrasing somebody else´s words above.
It does not matter if you are using all, some or none of the words used by the original author when rendering his or her ideas, you always need to credit that person by leaving a citation and a reference.
COPYING A PHRASE OR A SENTENCE FROM SOMEBODY ELSE WITHOUT QUOTATION MARKS AND A CREDIT
This is plagiarism, cf. above.
Quotation marks, in-text citation, and a reference are all mandatory elements when quoting somebody else´s work. No exceptions apply.
SUBMITTING (PARTS OF) YOUR OWN PREVIOUS PAPER AGAIN FOR A NEW COURSE WITHOUT FLAGGING IT UP
This is a case of self-plagiarism.
Using your own previous work in a paper poses the same issues and requires the same solutions as using somebody else´s work. You need to flag it up very clearly to the reader by way of a citation and reference. If you fail to do this, you will be plagiarising.
Plagiarising yourself is just as serious an offense as plagiarising somebody else.
Besides formal technical requirements such as observing the general referencing principles, there might also be other issues at play when re-using your own work.
Sitting an exam, whether oral or written, is in most cases a test of skills and knowledge acquired in a particular course. Re-using material from a different context to pass this test would then seem to go against the intention of the exam, which is to test course-specific knowledge and as such could be perceived as not only plagiarising but also plain cheating.
For that reason it is always a good idea to ask your supervisor for advice and, if needed, permission to resubmit part of previous work.
Submitting an entire paper again is obviously wrong from whichever perspective.