Reference

Common Confusions & Confusable Words

Here's the thing — most of these never make it as far as real grammar. They're just lookalike words that keep swapping places, usually in the email you fire off at 4:55 on a Friday. Spot the swap and you're done. One clean tell each, then a door through to the article that actually teaches it.

Pair The quick tell Learn it here
affect / effect Affect is the verb (to influence); effect is the noun (the result). P8 · Word choice & confusables
its / it's Its = belonging to it; it's = it is or it has. P2 · Pronouns & possessives
their / there / they're Their = belonging to them; there = a place; they're = they are. P2 · Pronouns
your / you're Your = belonging to you; you're = you are. P2 · Pronouns & contractions
who / whom Who does the action (subject); whom receives it (object) — swap in he/him to test. P2 · Pronouns
fewer / less Fewer for things you can count one by one; less for a mass you can't. P5 · Agreement & concord
then / than Then = time or what happens next; than = a comparison. P8 · Word choice & confusables
accept / except Accept = to receive or agree to; except = apart from. P8 · Word choice & confusables
principle / principal Principle = a rule or belief; principal = main, or the head of a school. P8 · Word choice & confusables
complement / compliment A complement completes something; a compliment praises it. P8 · Word choice & confusables
practice / practise Practice = the noun; practise = the verb (UK — see note below). P8 · Word choice & confusables
advice / advise Advice (with a c) = the noun; advise (with an s) = the verb. P8 · Word choice & confusables
to / too / two To = direction or a verb marker; too = also / excessively; two = the number 2. P8 · Word choice & confusables
lose / loose Lose = to misplace or not win; loose = not tight. P8 · Word choice & confusables
allusion / illusion An allusion is an indirect reference; an illusion deceives the senses. P8 · Word choice & confusables
stationary / stationery Stationary (with an a) = not moving; stationery (with an e) = paper and pens. P8 · Word choice & confusables
phrase / clause A clause has a subject and a verb; a phrase doesn't. P3 · Sentence structure & syntax

A quick UK / US note

Only one pair here is genuinely dialectal in the mix-up itself:

UK US
practice / practise practice = noun, practise = verb (rule) practice = both noun and verb (variant)

Everywhere else above, the same trap catches writers on both sides of the Atlantic — the quick tell holds regardless. For the fuller spelling and vocabulary picture, see the Master UK/US Index.

Hit a pair that isn't here? Head to P10 — it'll almost certainly be waiting for you.