Avoid these common errors when filling out your applications: https://insights.collegeconfidential.com/8-grammar-errors-make-admissions-officers-cringe
Now that it is acceptable to mix pronouns (to most of the world, not to me), this rule is hard to follow. If the writer is using ‘they’ instead of he/she, what verb is to be used? It all sounds very clumsy to me.
What does “mix pronouns” mean, @twoinanddone?
Overall, I think the advice to proofread super carefully is spot on.
The grammar advice a little less so. Or at any rate, it’s a little alarmist in a couple of cases - I doubt there are very many application readers who cringe even a tiny bit over a split infinitive, and very few over a preposition at the end of a sentence. They’re going to prefer writing that reads smoothly, not writing that’s contorted to fit rules that arguably don’t even apply to a piece of writing like a personal statement.
Use the correct word(s), and proofread carefully.
The article lists items that were repeatedly mentioned in a survey of actual admissions officers. Why do you doubt it’s true?
Why not just get it right?
“They” takes a plural verb even if it is a singular reference. It’s similar to how “you” always takes the plural form. “You is” is incorrect as is “they is.”
Who decides what is right? A sample size of 18 colleges is not exactly statistically significant As widely attributed to Churchill, "“This is the sort of bloody nonsense up with which I will not put.” 
That said, proofreading an essay,and having someone proof it after you should be standard before clicking the submit button on a college application.
+1 to @skieurope (as usual), for the sense and the always-worth-repeating quote.
@RichInPitt, take this with all the salt you like; I don’t mean anything unkind by it. But if colleges are actually dinging kids whose essays are otherwise well written (and carefully proofread) for split infinitives or the occasional well constructed sentence that just happens to end in a preposition, they’re not using good judgment. And I really can’t believe that very many actually are.
Language changes. Standards in places that I’m betting you respect have long since moved past the notion of following those two particular rules in any kind of hidebound, hard-and-fast way.* It doesn’t mean that a writer (applicant or otherwise) shouldn’t look at a sentence with a split infinitive to see IF there’s a better way to write it, or re-think a sentence that you’ve casually ended with a preposition to see whether it could work a different way. It means those constructions aren’t categorically wrong, and it’s a petty way to look at writing.
*And I do mean long since. Like, a lot of us on this board were still in college. Or maybe high school. “CMOS has not, since the thirteenth edition (1983), frowned on the split infinitive.” (Chicago Manual of Style) Just for one example.
Also from the CMOS website: “CMOS has never prohibited a preposition at the end of a sentence in any of its versions and editions since 1906. The first edition to state positively that a preposition may end a sentence was the 15th, in 2003, the first edition of the Manual to contain a chapter on grammar.”
They really mean it. 
CMOS 5.180: “The traditional caveat of yesteryear against ending sentences or clauses with prepositions is an unnecessary and pedantic restriction. And it is wrong.”
Many kids have trouble with grammar, proper sentence construction, etc. thanks to texting. It’s really a problem as they don’t recognize the informality of their communication medium being an issue to the rest of us. Really smart kids too. Writing is like anything else. It requires practice. Like a muscle, it needs to be used. They spend so much time texting or snapchatting or whatever with their own form of language, it becomes hard to bounce back to reality.
Guess that won’t matter when they rule the world, but it does now 
That’s good news to Trekies/Trekkers everywhere. 
“…grammatical pet peeves.” Not reason to reject.
And once again, we confer authority on a site that serves it’s own interests. They link to their pro counseling service.
See how easy it is to market to the worried?
I see what you did there…?
I wonder what AOs think of cliches like “The Bottom Line”?
It’s not a grammatical error, but certainly overused, and worthy of it’s own thread.
my peeves:
“apart” used incorrectly so often! “Apart of my activities include:” SO Wrong!
These students don’t seem to understand that “apart” (adv.) means to separate.
We took apart the engine in two hours.
“alot” used often
then (adverb) to indicate status of time: We ate, then we went to the movies.
than (conjunction) to indicate a comparison: I like this shirt rather than that one.
I agree that it’s not necessarily a reason to reject but it does seem like it’s not too difficult to ask someone else (such as a teacher or counselor) to proofread for you. Unfortunately small things like this can have an outsized effect on people whose job it it is to read piles of applications.
Same thing happens with resumes, by the way.
At the very base of human relations is language. If there are no rules there, then what? How may we all get across our thoughts and meaning succinctly and intelligently, together, without all following the same rules of language?
Lol, can’t imagine how “bottom line” has a place in an app essay.
I’ll admit these threads about grammar make me self-conscious.
“At the end of the day, the bottom line is that Harvard is my dream school.” 
@skieurope ???