Common Grammar Mistakes Explained
Most grammar errors that show up on essays fall into a short, predictable list. Learning to spot these few patterns catches a surprising share of the mistakes graders actually mark, well before you need to memorize a full grammar textbook.
Subject-Verb Agreement
A singular subject needs a singular verb; a plural subject needs a plural verb. The trap is usually a phrase sitting between subject and verb that seems to change the subject's number. In "The box of documents were misplaced," the writer's ear latches onto "documents" (plural) instead of the real subject, "box" (singular) — the sentence should read "The box of documents was misplaced." A reliable check is to mentally delete the prepositional phrase ("of documents") and see whether the verb still sounds right against the bare subject.
Misplaced and Dangling Modifiers
A modifier should sit next to the word it describes. "Running down the street, the bus was missed by Sarah" dangles "running down the street" with no clear subject performing that action — grammatically, it reads as though the bus was running. The fix is to give the modifier a subject to attach to: "Running down the street, Sarah missed the bus." A misplaced modifier is a related but distinct problem, where the describing phrase sits too far from its intended target: "She served sandwiches to the guests on paper plates" implies the guests were on paper plates; moving the phrase fixes it: "She served sandwiches on paper plates to the guests."
Pronoun Agreement and Reference
A pronoun must agree in number with the noun it replaces, and its reference must be unambiguous. "Every student must bring their own laptop" is now widely accepted with singular "they" for gender-neutral reference, but many style guides in academic writing still prefer "Every student must bring his or her own laptop" or, more smoothly, rewriting the sentence entirely: "Students must bring their own laptops." A separate issue is unclear reference: "Maria told Priya that she had gotten the highest score" leaves ambiguous who scored highest — the sentence needs to be rewritten to name the person directly.
Comma Splices and Run-On Sentences
A comma splice joins two independent clauses (each capable of standing as its own sentence) with only a comma: "The experiment failed, we had measured the wrong variable." Because a comma alone isn't strong enough to join two complete sentences, this needs one of several fixes: a period ("The experiment failed. We had measured the wrong variable."), a semicolon ("The experiment failed; we had measured the wrong variable."), or a coordinating conjunction added after the comma ("The experiment failed, because we had measured the wrong variable" — or, more naturally, "for we had measured the wrong variable").
Its vs. It's, and Other Commonly Confused Words
"It's" is always a contraction of "it is" or "it has"; "its" is the possessive form ("The company updated its policy," not "it's policy"). A useful test: if you can substitute "it is" into the sentence and it still makes sense, use the apostrophe. Similar confusions include "affect" (usually a verb, to influence) versus "effect" (usually a noun, a result), and "there," "their," and "they're," which are unrelated words that happen to sound identical and are frequently swapped by autocorrect as much as by the writer.
Parallel Structure in Lists
Items joined in a list or by a coordinating conjunction should use matching grammatical forms. "She enjoys hiking, to swim, and reading" breaks parallelism by mixing a gerund, an infinitive, and another gerund; the fix is to make all three match: "She enjoys hiking, swimming, and reading." This same principle applies to longer sentence structures, such as comparisons ("It is easier to write clearly than writing quickly" should be "easier to write clearly than to write quickly").
Summary
Subject-verb agreement, misplaced or dangling modifiers, pronoun reference, comma splices, easily confused words, and parallel structure account for the large majority of grammar corrections on a typical graded essay. Catching them reliably takes practice reading your own sentences slowly rather than skimming, ideally after setting a draft aside for even a short time. These skills pair directly with the planning covered in essay structure and planning and the sentence-level work in paragraph structure and unity. Purdue's Online Writing Lab (owl.purdue.edu) is a widely used free reference for working through additional examples of each of these patterns.