Affect vs. Effect, Then vs. Than, Its vs. It's

Some of the most commonly confused words in English are not rare or advanced at all. They are small, extremely frequent words that sound alike, look alike or both, and the confusion has little to do with how many words you know. It comes from how easily two nearly identical words can swap places in your ear. This guide takes three pairs, gives a rule and a quick test for each, and finishes with a short practice set.

Affect and effect

The two words are pronounced almost the same in speech, and they overlap in meaning, which is why they cause so much trouble.

The shortcut that handles most sentences: affect is usually a verb meaning to have an influence on something, and effect is usually a noun meaning the result of that influence. Cambridge Dictionary defines affect as to have an influence on someone or something, or to cause a change in someone or something, and defines the noun effect as the result of a particular influence.

• "The weather can affect your mood." (verb: influence)
• "The weather had a noticeable effect on the crowd." (noun: result)

A quick test: try replacing the word with influence (verb) or result (noun). If influence fits, you need affect. If result or impact fits, you need effect. "The new rule will ___ everyone" works with influence, so it is affect. "The new rule will have a big ___" works with result, so it is effect.

The exceptions: there are two. Effect can be a verb meaning to bring about or cause something to happen, as in "to effect change". That is a formal use and fairly rare. And affect can be a noun in psychology, meaning an emotion or mood as shown in someone's expression, which Cambridge Dictionary also lists. You will meet both mostly in formal or specialist writing. The simple verb and noun rule covers the ordinary cases.

A memory aid: affect and action both start with A, and affect is an action word. Effect and end result both start with E.

Then and than

These two are easy to mix up because they look alike and sound almost the same in fast speech. They never do each other's job.

Then is about time or sequence. Cambridge Dictionary lists meanings such as at that time, and after that. "I finished dinner, then went for a walk." "If you are ready, then we can start."

Than is used in comparisons. "This word is harder than that one." "She arrived earlier than I expected."

A quick test: if you are comparing two things, you need than. If you are describing what happened next or at what time, you need then. Another trick is to listen for the comparative word, such as more, less, better, worse, -er, rather or other. If one of those is in the sentence, than usually follows.

A memory aid: than and comparison share an A, and then and time share an E.

One more case: in "if... then" sentences, then introduces the result: "If it rains, then we will stay home." That is still a sequence, because the second part follows from the first.

Its and it's

This pair is the most common source of errors among writers, native and non-native alike. The usual apostrophe logic fails here, which is exactly why it trips so many people.

It's is always a contraction. It stands for it is or it has. "It's raining." "It's been a long week."

Its is a possessive, meaning belonging to it. "The dog wagged its tail." "The company changed its name."

The confusion arises because possessives in English often take an apostrophe (Maria's book, the dog's tail), so writers carry the habit into its. But possessive pronouns, such as his, hers, ours, theirs and its, never take an apostrophe.

A quick test: replace the word with it is or it has. If the sentence still makes sense, use it's. If not, use its. "The cat licked ___ paw" fails with it is paw, so it is its. "___ time to go" works with it is time to go, so it is it's.

A memory aid: it's has an apostrophe because a letter has gone missing. The apostrophe replaces the i in is.

Why these pairs stay sticky

All three pairs share a feature. They are frequent enough that you use them constantly, so a mistake gets repeated and reinforced instead of corrected. And they sound almost identical in speech, so your ear never flags the difference the way it would for two words that sound distinct. Reading and writing, not listening, is where these errors get caught.

That is also why simply reading a rule once rarely fixes the habit. The rule has to be applied several times in real sentences, with feedback, before the right choice becomes automatic.

A practice set

Choose the correct word in each sentence. Answers follow.

1. The new policy will (affect / effect) all departments.
2. Exercise has a positive (affect / effect) on sleep.
3. She is taller (then / than) her brother.
4. We had lunch and (then / than) went back to work.
5. The company announced (its / it's) results yesterday.
6. (Its / It's) important to check your work.
7. The noise did not (affect / effect) his concentration.
8. If you finish early, (then / than) you can leave.

Answers: 1. affect. 2. effect. 3. than. 4. then. 5. its. 6. It's. 7. affect. 8. then.

If you missed any, look at the quick test for that pair and apply it again to the sentence. Doing that is more useful than rereading the rule.

A method for making it automatic

Choose one pair at a time. For a week, write down every sentence you see or write that contains one of its words, and apply the quick test. Add the sentences you got wrong to a short list, and test yourself on the list two days later and a week later. Spaced testing works better than rereading a rule, as how spaced repetition works and active recall versus rereading explain.

When you proofread your own writing, search for each of these words and check them one by one. A search for "its", "it's", "then", "than", "affect" and "effect" in a document takes under a minute and catches most errors. Spellcheckers usually do not, because both words in each pair are real words. Complement vs. compliment covers another set that spellcheck misses, and imply vs. infer covers a pair where the meaning, not the spelling, is the problem.

How Wordabulary fits in

Wordabulary shows each word with a definition and an example sentence, which is the best way to see how a word behaves, and quizzes include a typing format that asks you to produce words. You can look up affect and effect in the word library and test yourself on both. It works best alongside writing practice and proofreading your own text.