In writing a phd thesis, if a sentence is copied as it is and given proper references, then will it be considered as plagiarism?
This can reach levels of silliness.
If Hunter and Garcia did a study on aadvark toenails, and wrote in 2008 that "We sampled 200 aardvarks and we conclude that all aardvarks have long toenails."
I would be satisfied with:
Hunter concluded that all aadvarks have long toenails (Hunter and Garcia, 2008).
I would also be fine with:
All aardvarks have long toenails (Hunter and Garcia, 2008).
I have had TAs report cases of both types to me as plagiaristic. I simply don't agree. There is no reason to change a simple sentence that has a limited number of ways it can be phrased to
All aadvarks have toenails of great length (Hunter and Garcia, 2008)
to avoid charges of plagiarism.
Sometimes, because there are limited ways to write out a phrase, an attempt to paraphrase results in sentences that have parts that come out the same. So long as citation is proper, I don't believe quotes are necessary.
Though this has little to do with plagiarism, on undergrad papers in the hard sciences, I often see students going to great lengths to include quotes, because they believe it to be stylistically required for some reason, but in the hard sciences, paraphrasing is the norm, and overquoting just looks silly. Our writing folks tell me its not this way across all disciplines.
Going back to plagiarism, and all may not agree, I try to determine if the author is trying to represent someone else's work as their own. For the examples I cite, clearly not, and thus they're not examples of plagiarism.
Where do I draw my line? Probably at full sentences or large phrases, where it's appropriate to use quotation marks. In the hard sciences, paraphrasing is usually more appropriate anyway.
Where does one use quotes in the hard sciences, then?? When the phrasing is important or has significance.
Einstein said "In the middle of difficulty lies opportunity"
is a different passage than
In Einstein's opinion, one may often find opportunities when difficulties are apparent.
To wrap up, if you are copying a sentence, it is most likely appropriate to use quotes, but in many fields the accepted style is such that it is not appropriate to copy a sentence.