The tweeter-in-chief wouldn’t pass third grade 

This error suggests that the president of the United States either (a) shoots off his tweets somewhat hastily, or (b) fails to proofread everything he writes, or (c) does not understand the difference between singular and plural nouns, or (d) all of the above. 
The failure to proofread everything one writes is widespread in the modern world, and possibly even in the unmodern one. As a former teacher of high-school English, I can testify that my biggest headache in 30 years of teaching was the failure of students to proofread their written compositions. Repeated pleas, exhortations, tirades, tantrums, and threats of flogging, crucifixion, and boiling in oil made not a dent in the impenetrable wall of student recalcitrance.
This is not the first error that the president has made in his lengthy history of tweetstorms. Alas, advanced age and the repeated assaults on my brain cells caused by decades of teaching have brought about my failure to document those sins.
But this latest instance – “81 letter”! – should suffice to convict the president of either (a) carelessness or (b) incompetence or (c) both. This should be an impeachable offence. It is further proof that this president does not keep his inner third-grader in check. 
To save him further embarrassment, would it not be wise to employ a presidential proofreader to screen his tweets for errors before he shoots them off? I would be willing to offer my services in exchange for a cabinet-level position and a modest six-figure salary (seven figures would also be okay).
But the purpose of this letter is not to mount a crude hustle for a job. It is to chide the president for his carelessness/incompetence and urge him to repent and reform. Baaaaad Donnie! Go sit in the corner till recess time.
Ye Really Olde Pedagogue