What to do when you think a student is incapable of using certain vocabulary? I think many here have had the experience of seeing a paper that uses grammar dramatically different than its student author has shown in the past, and naturally questioning the authorship follows. But what about when the student uses a single word or short phrase that is out of character? Is that sufficient to justify doubting the authorship? Is it appropriate to suspect plagiarism based on correct usage of a single word by a student who never (in your direct experience) used this word/phrase before? Should we make allowances for age (do older people know more words?) or native language (does this improve or diminish vocabulary?) in deciding the likelihood of the word being a red flag for (mis)appropriated work? 

The above question is inspired by the incident described in this blog post by an undergraduate student at Suffolk University. In this blog post the student describes an incident where a professor negatively challenged her vocabulary skills and proceeded to publicly humiliated her in front of her classmates:


  This morning, my professor handed me back a paper (a literature review) in front of my entire class and exclaimed “this is not your language.” On the top of the page they wrote in blue ink: “Please go back and indicate where you cut and paste.” The period was included. They assumed that the work I turned in was not my own. My professor did not ask me if it was my language, instead they immediately blamed me in front of peers. On the second page the professor circled the word “hence” and wrote in between the typed lines “This is not your word.” The word “not” was underlined. Twice. My professor assumed someone like me would never use language like that.


Ordinarily some might assume this is just the griping of an unhappy undergrad, but the article starts with the student describing her bona fides which are rather impressive (URL link added by me):


  As a McNair Fellow and student scholar, I’ve presented at national conferences in San Francisco, San Diego, and Miami. I have crafted a critical reflection piece that was published in a peer-reviewed journal managed by the Pell Institute for the Study of Higher Education and Council for Opportunity in Education. I have consistently juggled at least two jobs and maintained the status of a full-time student and Dean’s list recipient since my first year at Suffolk University. I have used this past summer to supervise a teen girls empower program and craft a thirty page intensive research project funded by the federal government. As a first generation college student, first generation U.S. citizen, and aspiring professor I have confronted a number of obstacles in order to earn every accomplishment and award I have accumulated. In the face of struggle, I have persevered and continuously produced content that is of high caliber.


Let me be clear that in my asking this Question and others providing an Answer it is important for everyone here to do two things: 


Ignore the shameful behavior of the instructor, and 
Ignore the student's opinion about her instructor's bias. 


I am not saying that these two issues are unimportant, they definitely are, but neither of these issues is the focus of this question. The key question I want to ask is expressed in the title: "What to do when you think a student is incapable of using certain vocabulary?" I use this blog post as an example of the situation that may arise and need to be dealt with.

If I see such a one-word red flag should I speak with the author and express my concern that it makes the work suspect? If so then how do I balance that against the fact that (a) it is a given that students doing academic research are exposed to academic language and are likely to acquire some of this vocabulary by sheer osmosis, and (b) isn't there a danger of discouraging students from expanding their vocabulary if use of new words creates negative reactions?
Thesauri are a thing.  The internet is a thing.  As such, everyone is capable of using all vocabulary and dialects at all times. They're also capable of changing their minds about how they express themselves at any moment.  One could write a serious mathematics paper entirely in Middle English (modulo terminology a bit, which will be hard to adapt, unless the mathematics is historically appropriate to the time) if they were so inclined. 

If the words are used correctly in the student's work, then in the absence of actually substantive evidence of plagiarism or academic dishonesty, then there's nothing to do.  At best you can comment on whether the choice of phrasing is suitable for the topic or not, and if it helps or hinders the clarity of the exposition.  That Middle English math paper is fairly unsuitable and inappropriate, and almost certainly unspeakably unclear, unless, say, the assignment was specifically concerned with writing in Middle English (in which case maybe the math is the inappropriate and unclear thing).

Furthermore, academic writing is often a dialect of its own.  In mathematics we often say things like "We will show..." even in single author papers, because that's the history and convention.  It is entirely reasonable that efforts at serious academic writing will not sound like natural conversation, and that anyone familiar with how a person sounds in only one of those will be surprised by how they sound in the other.