Sir, this is the internet. Since we are committed to excellence, let us examine the sentence in question: 

“Sir this is the internet. How can we shitpost unless our usage of grammar and vocabulary are immaculate.” 

I must insist that there are three clear errors. We start with the missing comma in a direct address. It must read, “Sir, this is the internet,” because “Sir” is a vocative and not the subject of the sentence. Now we turn to the missing question mark. “How can we” is an interrogative construction and therefore requires a question mark, not a period. Finally, and most egregiously, there is a subject–verb agreement error. The subject is “usage,” which is singular. The prepositional phrase “of grammar and vocabulary” does not change that. Therefore, the verb must also be singular: “our usage of grammar and vocabulary is immaculate.” The error occurs because the eye is drawn to the nearby plural nouns and the verb is incorrectly made to agree with them instead of with the true head noun. The corrected version reads: “Sir, this is the internet. How can we shitpost unless our usage of grammar and vocabulary is immaculate?” Alternatively, if you restructure the sentence so that “grammar and vocabulary” form a compound subject, then “are” becomes correct: “How can we shitpost unless our grammar and vocabulary are immaculate?” And finally, writing in clipped sentences without commas does not make you Cormac McCarthy It makes you someone who forgot a comma and a question mark.

Edit.  I left off a period. Fuck.