You’ve written a company newsletter, run the spell checker, and a colleague still circles “its” in red because it should have had an apostrophe. Spell checkers miss these slips because every word involved is spelled correctly. This guide covers five confusing word pairs that trip up careful writers (one of them is a trio), with a rule, a quick test and a worked example for each, plus a routine for catching them in your own drafts.
Five confusing word pairs at a glance
The table gives the rule and a test you can run in a few seconds. The sections below explain each in more detail, with the exceptions that catch people out.
| Pair | Rule | Quick test | Wrong, then right |
|---|---|---|---|
| Affect / effect | Affect is usually a verb; effect is usually a noun | Swap in “change” or “result” | The delay effected our plans. / The delay affected our plans. |
| Its / it’s | Its shows possession; it’s means “it is” or “it has” | Read it as “it is” | The firm changed it’s policy. / The firm changed its policy. |
| Then / than | Then is about time or sequence; than is for comparison | Is something being compared? | The second draft was shorter then the first. / …shorter than the first. |
| Their / there / they’re | Their shows ownership; there shows place; they’re means “they are” | Read it as “they are” | Their going to review it over there. / They’re going to review it over there. |
| Fewer / less | Fewer goes with things you count; less goes with amounts | Can you put a number in front of the noun? | We have less errors now. / We have fewer errors now. |
Affect and effect
This is the pair with the most exceptions, so it’s worth learning the exceptions. Touro University’s writing centre puts the default rule plainly: “Generally, we use affect as a verb (an action word) and effect as a noun (an object word).” It defines affect as to act on or produce a change in, and effect as a result or consequence.
Its tests are easy to apply. If you could swap in another verb such as “damaged” or “moved”, affect is correct. If you could swap in “result”, effect is correct. The same page notes that effect often follows words like “the”, “any”, “an”, “into”, “on” and “take”, and affect doesn’t.
Example: “The new schedule will affect the whole team” (verb: it changes them). “The new schedule had a positive effect on the team” (noun: the result).
The two exceptions
Effect can be a verb meaning to bring about, as in “The new manager is bound to effect positive changes in the office”. Affect can be a noun meaning a display of emotion. Both are uncommon in everyday writing. If you’re ever unsure, rewrite the sentence with a plainer verb, such as “cause” or “change”, and skip the exception.
Its and it’s
The apostrophe trap is that apostrophes normally mark possession (the writer’s draft, the team’s goal), so “it’s” looks as if it should mean “belonging to it”. It doesn’t. Vanderbilt University’s writing studio handout on apostrophe trouble labels “The cat hurt its paw” as possessive and “It’s the cat purring” as a contraction of “it is”. Its test is to rephrase the word as “[blank] is”, which requires the apostrophe. If “it is” fits, write “it’s”; if not, write “its”.
Reed College’s writing lab gives a matching example of the error: “A coniferous tree continually sheds it’s leaves” is wrong, and “its leaves” is right. It also offers a second check: try “his”. If “his” fits, as in “sheds his leaves”, use “its”.
Example: “The report is late, and it’s [it is] the third time” is right. “The report is late, and its [belonging to it] cover page is missing” is also right. “It’s cover page” would read as “it is cover page”, which makes no sense.
One extra case: “it’s” also stands for “it has” when it comes before a past participle, as in “it’s been a long week”. The “it is” test fails there, so read it as “it has” as well.
Then and than
The rule is the shortest of the five. Towson University’s Online Writing Support page says “then” usually refers to time, meaning “at that time”, and can also be a transition meaning “in that case” or “therefore”. It also says “than” is used only when comparing items, and that “then” is never used in comparisons.
That gives you a one-question test: is anything being compared? If yes, write “than”. The same page offers a memory hook, linking “comparing” and “than” through the letter a.
Example: “Our second draft was clearer than the first, and then we sent it out.” The first clause compares two drafts, so it takes “than”. The second describes what happened next, so it takes “then”.
Watch the phrases “rather than”, “other than” and “more than”, which always take “than”. And in “if you’re ready, then we can start”, “then” means “in that case”. Neither is a comparison.
Their, there and they’re
This is a trio rather than a pair, but the confusion works the same way. Champlain College’s writing centre defines the three words as follows: “their” is a possessive pronoun, “there” indicates direction or location, and “they’re” is a contraction of “they are”. Its examples are “We are about to pass their house”, “You can set the boxes down over there” and “They’re supposed to be finished with the job by 5 p.m.”
The easiest test is the same as the one for it’s: say the sentence with “they are”. If it still works, write “they’re”. If it doesn’t, ask whether you mean ownership (their) or place (there). The word “there” also begins sentences with a form of “to be”, as in “There are three options”, where it names no place at all.
Example: “They’re sure their data is stored over there.” The first word is “they are”, the second shows ownership of the data, and the third points to a location.
Fewer and less
The Collins Dictionary language blog states the core rule: use “less” in front of an uncountable noun, as in “A shower uses less water than a bath”. It adds that although some people think it is wrong, “less” can sometimes be used in front of plural nouns, especially in conversation, but that in formal and written language you should use “fewer” in front of plural nouns, as in “There are fewer trees here”.
So the test is whether you can put a number in front of the noun. You can count trees, errors and drafts, so those take “fewer”. You can’t count water, traffic or confidence, so those take “less”.
Amounts of money and distance are the exceptions that cause trouble. The same Collins post gives “It’s hard to find a house in Beverly Hills for less than a million dollars” as a natural use of “less than”, with a distance example alongside. It also says that in formal writing you should use “fewer than”, not “less than”, with people or things.
Example: “Fewer than 50 readers replied, which means we got less feedback than last month.” Readers can be counted, so “fewer than”; feedback is treated as an uncountable amount, so “less”.
How to catch these slips in your own draft
A spell checker won’t flag any of the five pairs, so the catching has to be done on purpose. These steps cost about ten minutes on a 1,000-word article.
- Finish the draft first. The University of North Carolina’s writing centre says not to proofread as you go, because you may end up cutting the sentence you’re polishing.
- Take a break. The same handout says to wait awhile after finishing a draft, and that a day, or even a few hours, will work.
- Search for the problem words. Use the find tool for “its”, “it’s”, “than”, “then”, “their”, “there”, “they’re”, “affect”, “effect”, “less” and “fewer”. It’s a short list, and you’ll check each instance in seconds.
- Apply the test. Swap in “it is” or “they are”, ask whether something is being compared, and try putting a number before the noun.
- Read aloud. The UNC handout recommends reading the paper out loud as one way to see how well things flow, and notes that sentence by sentence you’ll notice where you stumble.
Test yourself
Pick the right word in each sentence, then check the answers below.
- The new rule will (affect / effect) every department.
- The cat licked (its / it’s) paw.
- Prices are higher (then / than) they were in spring.
- (Their / There / They’re) is a typo on page two.
- There were (fewer / less) complaints this month.
Answers: affect (a verb meaning to change), its (possession), than (comparison), There (the sentence opens with “to be” and names no owner), fewer (complaints can be counted).
Common mistakes
Trusting autocorrect and spell check. Every word in the table is a real word. The tools see no spelling error, so only a person checking the meaning can catch the slip.
Memorising a trick and applying it blindly. The “affect is a verb” shortcut breaks on the two exceptions, and “less is for uncountable nouns” breaks on money and distance. When a rule feels shaky, rewrite the sentence.
Over-correcting. Writers who learn the fewer/less rule sometimes write “fewer” in places where “less” is correct, such as “fewer than $50”. The Collins post’s own money example uses “less than”. Check whether the amount is counted as separate items or treated as one sum.
A quick reference checklist
- Affect: can you swap in “change”? Effect: can you swap in “result”?
- Its or it’s: does “it is” (or “it has”) fit? If yes, use the apostrophe.
- Then or than: is anything being compared? If yes, than.
- Their, there or they’re: does “they are” fit? If not, is it ownership or place?
- Fewer or less: can you put a number in front of the noun?
- Searched the draft for all of these words before sending
- Read the final version aloud
A spell checker knows whether a word exists. Only the writer knows whether it’s the word that was meant.
The takeaway
Five rules cover most of the trouble: affect acts and effect results, it’s means “it is”, than compares and then sequences, they’re means “they are”, and fewer is for things you can count. Search your draft for the problem words after you finish writing, test each one, and read the result aloud. The exceptions are rare enough to look up when they come up.
