BK Writing Lab

Common Grammar Errors
Paul DiGeorgio, Ph.D.


Here are several common grammar mistakes that you should avoid in your writing.


they're = they are

e.g. They're going to school.

their = possessive

e.g. They're going to realize that they forgot their books before they get to school.

there = often an adjective or adverb

e.g. They're probably going to arrive there and then realize they forgot their books.

then vs. than

'Than' should be used for comparisons.

e.g. The dog is larger than the cat. 

'Then' is a temporal word, meaning that it has to do with time. The word is also often used for cause and effect.

e.g. If you do not like cats, then why did you get one?

its vs. it's

'Its' is possessive. 

e.g. The computer is overheating its graphics card.

'It's' is a contraction of 'it is.'

e.g. It's a shame that we couldn't win that last football game.

good vs. well

'Good' is an adjective or a noun. When the word is an adjective it can only modify a noun. An adjective cannot modify a verb or another adjective.

e.g. You did a good job on the exam! (adjective)

e.g. I hope that all of my students go out into the world and do good (noun -- in other words, the students will hopefully do good things.)

'Well' is an adverb. Therefore it modifies a verb, an adjective, another adverb, or a sentence or clause.

e.g. I am not feeling well today! (adverb -- here the adverb is modifying the verb 'feeling')

Note: When someone asks you how you are doing, you should respond with "well" (if you are well) and not with "good" -- see the example above!

misplaced and dangling modifiers

A misplaced modifier is a word that is placed in the wrong position and therefore is modifying the wrong word in a sentence. 

e.g. We fed berries to the birds in little cups. [This makes it sound like the birds are in little cups. Fix: We fed the birds berries in little cups.]

A dangling modifier is a word that is modifying ambiguously. Sometimes there are multiple words in the sentence that could be modified, while at other times a dangling modifier is trying to modify a word that is not in the sentence.

e.g. Without knowing the formula, it was impossible to solve the equation. [The issue is that we do not know who did not know the formula. While lots of people might talk like this, this is bad grammar and you should avoid this in your writing.] 

sentence fragments

A sentence fragment is the same thing as an incomplete sentence. At the bare minimum, every sentence needs to have a subject and a verb.

Typically students commit this error when they do not pair a dependent clause with a main/independent clause.

e.g. Whether it's deciding to admit a mistake or being afraid to say another person is right. [This is incomplete. We would need to add to this dependent clause in order for this to be a grammatical sentence.]

run-on sentences

A run-on sentence is a sentence that lacks the correct punctuation or conjunction. Typically a run-on sentence includes multiple independent clauses that "run together" instead of being linked by the proper punctuation or conjunction.

Typically a student can fix a run-on sentence by using a conjunction like 'and' or punctuation like the semicolon ( ; )

e.g. I have four sisters the oldest is named Julia. [Fix: I have four sisters. The oldest is named Julia. OR I have four sisters; the oldest is named Julia. OR I have four sisters and the oldest is named Julia.]

commas and semicolons

Often students use commas in situations that require semicolons. 

e.g. He told me that he was going to the store, he asked if I want anything. [This is bad grammar. You have to use a semicolon to link these two clauses. You cannot use a comma.]

agreement errors

Your subject and verb need to agree in number. This means that you need to use the right form of a verb depending on your subject. 

e.g. The air conditioning in those rooms work poorly. [Here the subject of the sentence is 'air conditioning' so the verb should be 'works.' The student probably mistakenly used the singular verb form 'work' because he or she was confused by the noun 'rooms' but that is not the subject of the sentence.]


If you have any questions, send us an email at bkwritinglab@bishopkennyhs.org!


Image Credit: Error by Adrien Coquet from the Noun Project

  • Grammar
  • Mistakes
  • Writing