Commonly confused words that cost marks in formal writing
8 min read · Updated 7 October 2026
Quick take. A handful of word pairs cause most wrong-word errors in formal writing: affect (verb) and effect (noun), rise (no object) and raise (takes an object), fewer (countable) and less (uncountable), economic (about the economy) and economical (saves money), advice (noun) and advise (verb), lose and loose, and its and it's. Learn the one-line test for each below, then check your own writing for the three you get wrong most often. You do not need the whole list, just your own pairs.
Lists of confused words often run to a hundred pairs, most of which nobody actually mixes up. This guide keeps to the ones that turn up again and again in essays, applications and formal emails, and gives one quick test for each so you can check a sentence in seconds.
Why it matters: in IELTS Writing, a wrong word counts against Lexical Resource, and a wrong verb form against Grammatical Range and Accuracy. In an application or an email to a professor, it is the kind of slip a reader notices first.
The pairs that matter most
| Pair | The difference | Quick test |
|---|---|---|
| affect / effect | Affect is usually a verb; effect is usually a noun. | If you can put "the" before it, use effect. |
| rise / raise | Rise has no object (prices rise); raise needs one (the government raises taxes). | Ask "raise what?" If there is an answer, use raise. |
| fewer / less | Fewer for things you count; less for amounts. | Can you put a number in front? Then fewer. |
| number / amount | Number for countable things; amount for uncountable. | Same test: countable means number. |
| economic / economical | Economic is about the economy; economical means saving money. | "Economic growth", but "an economical car". |
| advice / advise | Advice is the noun; advise is the verb. | You give advice; you advise someone. |
| practice / practise | In British English, practice is the noun and practise the verb. | "Medical practice", but "practise every day". |
| lose / loose | Lose is the verb (lose a job); loose is the adjective (loose clothing). | If it is something you can do, it is lose. |
| its / it's | Its shows possession; it's means "it is" or "it has". | Replace with "it is". If the sentence breaks, use its. |
| their / there / they're | Their shows possession; there a place or "there is"; they're means "they are". | Replace with "they are" to test they're. |
| then / than | Than compares; then is about time or order. | Comparing? Use than. |
| principal / principle | Principle is a rule or belief; principal means main, or a head of school. | "The main reason" means principal. |
| ensure / insure | Ensure means make certain; insure is about insurance. | No insurance policy involved? Use ensure. |
| accept / except | Accept means receive or agree; except means not including. | "Everyone except me". |
| complement / compliment | Complement completes or goes well with; compliment is praise. | Praise has an i: compliment. |
Pairs that come from everyday English
Some pairs are mixed up mostly because everyday spoken English in many countries uses them differently from formal written English. None of these is a sign of poor English. They are simply not what an examiner, admissions tutor or employer expects to read.
| Instead of | Formal written English | Why |
|---|---|---|
| Can you borrow me your book? | Can you lend me your book? | You lend to someone; you borrow from them. |
| He said me the answer. | He told me the answer. / He said the answer to me. | Tell takes a person directly; say does not. |
| Beside the cost, there are other problems. | Besides the cost, there are other problems. | Besides means "as well as"; beside means "next to". |
| This is specially important. | This is especially important. | Especially means "above all"; specially means "for a particular purpose". |
| Let us discuss about the problem. | Let us discuss the problem. | Discuss takes no "about". |
| Prices have increased to a large extend. | Prices have increased to a large extent. | Extent is the noun; extend is the verb. |
| The course is too much expensive. | The course is much too expensive. / It is very expensive. | "Too much" goes before a noun: too much money. |
British or American: pick one and stay with it
UK, Australian and many Commonwealth readers expect British spelling; US universities expect American. Both are correct, but switching between them in one essay looks careless. IELTS accepts either, as long as you are consistent.
| British | American |
|---|---|
| practise (verb), practice (noun) | practice (both) |
| licence (noun), license (verb) | license (both) |
| organise, recognise | organize, recognize |
| colour, behaviour | color, behavior |
| travelled, cancelled | traveled, canceled |
| programme (a course), program (software) | program (both) |
How to stop making the same mistake
Reading a list once will not change your writing. What works:
- Find your own pairs. Look through your last three pieces of writing, or an essay a teacher has marked, and note which pairs actually appear as mistakes. Most writers have two or three.
- Learn the quick test for just those, and use it every time you edit.
- Edit for them on purpose. After writing, search your text for each word in your pairs and run the test. It takes a minute.
If you are preparing for IELTS, wrong-word slips like these are among the easiest marks to win back. The essay checker lists clear grammar, spelling and word errors in your own essay, alongside an estimated band for each criterion. For the grammar slips that cost the most, see the common grammar mistakes guide.
Put it into practice
Write an essay and get an estimated band for each criterion, with the sentences to fix marked.