If you grew up speaking Korean and learned English later, you've probably noticed that certain mistakes keep showing up in your writing even after you've studied the rule behind them many times. That's completely normal: Korean and English have genuinely different grammatical structures, so some English rules don't map onto anything in your native language at all. Korean has no articles (a/an/the), no subject-verb number agreement, and different ways of marking time and aspect. The twenty patterns below are the ones that Korean learners run into again and again. After reading through them, try pasting your own writing into Dagochim's free English Grammar Checker to see these patterns flagged in your own sentences.
1. Articles: a vs. an
Wrong: I saw a elephant. Right: I saw an elephant.
The choice between a and an depends on the sound that follows, not the letter. If the next word starts with a vowel sound, use an; if it starts with a consonant sound, use a. That's why we say "an hour" (the h is silent) but "a university" (the u sounds like "yoo", a consonant sound).
2. Third-person singular -(e)s
Wrong: She go to work early. Right: She goes to work early.
When the subject is he, she, it, or a singular noun, present-tense verbs need an -(e)s ending. Since Korean verbs don't change form based on the subject's number or person, this ending is easy to forget, especially in longer sentences where the subject and verb are far apart.
3. There is vs. there are
Wrong: There is many reasons. Right: There are many reasons.
The verb after "there" has to agree with the noun that follows it, not with the word "there" itself. If that noun is plural, use "there are"; if singular, use "there is". Many learners default to "there is" out of habit and skip checking the noun's number.
4. Much vs. many
Wrong: I have much friends. Right: I have many friends.
Use much with uncountable nouns (money, time, water, information) and many with countable plural nouns (friends, books, people, ideas). A quick test: if you can count it one by one and make it plural, use many.
5. A few vs. a little
Wrong: I need a few advice. Right: I need a little advice.
A few goes with countable plural nouns (a few books, a few friends), while a little goes with uncountable nouns (a little advice, a little money, a little time). "Advice" is one of the uncountable nouns that often surprises Korean learners, since its Korean equivalent (조언) can be counted more freely.
6. Since vs. for
Wrong: I have lived here since five years. Right: I have lived here for five years.
Since marks a specific starting point ("since 2020," "since last Monday"), while for marks a span of duration ("for five years," "for two hours"). A simple way to check: if you could answer the question "since when?" with your phrase, use since; if you'd answer "for how long?", use for.
7. Make vs. do
Wrong: I did a mistake. Right: I made a mistake.
Make and do both roughly translate to "하다" in Korean, but English pairs them with specific nouns in ways that don't follow a simple rule; you largely have to memorize the collocations. Common ones include make a mistake, make a decision, make an effort, make a plan, but do homework, do the dishes, do research, and do exercise. When in doubt, it helps to learn these as fixed two-word chunks rather than translating word by word.
8. Double negatives
Wrong: I don't have no money. Right: I don't have any money.
In Korean, piling on negative words can emphasize a negative meaning, which can carry over into English as a habit. But standard English generally uses only one negative per clause. If you already have a negative verb form like don't, didn't, can't, or isn't, switch any following "no / nothing / nobody" to "any / anything / anybody".
9. Its vs. it's
Wrong: Its a great idea. Right: It's a great idea.
Its (no apostrophe) is the possessive form of it, meaning "belonging to it." It's (with an apostrophe) is a contraction of "it is" or "it has." Because they sound identical when spoken, this mix-up is purely a writing issue; reading your sentence aloud and mentally expanding "it's" to "it is" is a reliable way to check.
10. Your vs. you're
Wrong: Your welcome. Right: You're welcome.
Your is possessive ("your book"), while you're is a contraction of "you are." The classic phrase "you're welcome" is actually a shortened form of "you are welcome," which is a good way to remember the correct spelling.
11. Then vs. than
Wrong: This book is better then that one. Right: This book is better than that one.
Then describes a sequence in time ("first we ate, then we left"), while than is used for comparisons ("better than," "more expensive than," "rather than"). If you're comparing two things, you almost always want than.
12. Affect vs. effect
Wrong: This will effect the result. Right: This will affect the result.
Affect is almost always a verb meaning "to influence," while effect is almost always a noun meaning "a result." A handy trick: remember the phrase "have an effect on," which uses effect correctly as a noun, and reach for affect whenever you need a verb.
13. Prepositions of time: in, on, at
Wrong: I will meet you in Monday. Right: I will meet you on Monday.
English uses different prepositions depending on how specific the time reference is: in for years and months (in 2024, in May), on for specific days and dates (on Monday, on May 5th), and at for precise clock times (at 3pm). This three-way distinction doesn't exist in Korean time expressions, so it takes deliberate practice to internalize.
14. Present perfect vs. simple past
Wrong: I have seen him yesterday. Right: I saw him yesterday.
Present perfect (have/has + past participle) is for actions connected to the present without a specific finished time attached. Once you mention a specific past time word like yesterday, last week, or in 2019, you need the simple past tense instead, because the present perfect and a specific past time marker don't combine in standard English.
15. Verb tense inside if-clauses
Wrong: If I will have time, I will call you. Right: If I have time, I will call you.
Even when you're talking about a future possibility, the if-clause (the conditional part) uses the present tense, not will. The will goes in the main clause that describes the result. This pattern, sometimes called the "zero/first conditional," trips up many learners because the whole sentence is logically about the future.
16. Confusing noun and adjective forms
Wrong: This is very convenience. Right: This is very convenient.
Many English word families have distinct noun and adjective forms (convenience/convenient, importance/important, difference/different), and it's easy to grab the wrong one, especially after "very" or "be," which call for an adjective. When in doubt, check whether the word is naming a thing (noun) or describing a quality (adjective).
17. Double comparatives
Wrong: This is more better. Right: This is better.
Words like better and worse are already comparative forms of good and bad, so adding "more" in front of them is redundant and incorrect. The same logic applies to other irregular comparatives; if the word already means "more X," you don't need an extra "more."
18. Mixing up past tense and past participle
Wrong: The window was broke by the wind. Right: The window was broken by the wind.
The passive voice is formed with be + past participle, not be + simple past. For irregular verbs like break (broke/broken), write (wrote/written), and eat (ate/eaten), the past tense and past participle forms are different, and only the past participle works correctly after "be" in a passive sentence.
19. Base verb after a preposition
Wrong: I'm interested in learn English. Right: I'm interested in learning English.
After a preposition (in, about, of, for, and so on), English requires a gerund (the -ing form), not a bare infinitive. This is a very common slip because "interested in learn" can feel natural if you're translating directly from a Korean sentence structure.
20. Capitalization, spacing, and punctuation
Wrong: i think so ,but i'm not sure. Right: I think so, but I'm not sure.
Sentences should start with a capital letter, and the pronoun "I" is always capitalized, no matter where it appears. Punctuation marks like commas and periods should be directly attached to the word before them (no space before), with a single space after. These small formatting details are easy to overlook but make writing look noticeably more polished.
How to check these patterns in your own writing
- Open the English Grammar Checker.
- Paste in the example sentences above, or better yet, paste in something you wrote recently.
- Issues like article mistakes, agreement errors, much/many, since/for, make/do collocations, double negatives, and its/it's confusion are flagged instantly, each with a short explanation of why it's an issue.
- If you want a more natural rewrite of a sentence, click "Polish with AI" to see a before/after comparison powered by a small open-source language model running right in your browser.
A note on limitations
No free, privacy-first, browser-only tool can replace a human editor or a large commercial grammar service for every situation. The rule-based layer in this checker is intentionally focused: it targets well-documented patterns that Korean speakers commonly struggle with, rather than attempting to understand full paragraphs the way a large language model trained on millions of documents would. That focus is also its strength: it is fast, free, transparent about what it checks, and never requires sending your writing anywhere. Use it as a quick first pass and a learning tool for the patterns above, and give your important writing a final human read-through when it matters.
Frequently asked questions
Can a grammar checker catch every mistake?
No. Dagochim's English Grammar Checker uses rule-based patterns and curated word lists in its always-on Layer 1, so it focuses on well-known mistake types rather than every possible grammar issue. It is not as comprehensive as a paid service like Grammarly, but it is quite useful for the specific patterns Korean speakers commonly get wrong.
How do I tell since and for apart?
Use since with a specific starting point in time, like since 2020, and for with a duration, like for three years. Using since with a duration (since three years) or for with a point in time (for 2020) is a common mistake.
Is my text stored on a server?
No. Dagochim's English Grammar Checker runs its rule-based check entirely inside your browser and never sends your text to a server.