Why these are harder to catch than typos
Every word in this list is spelled correctly on its own, so spell-checkers have nothing to flag. The error is picking the right correctly-spelled word for the meaning intended, which is a word-choice mistake rather than a spelling mistake, and it survives careful proofreading more often than an outright typo would.
Some distinctions are fading in casual use
A few of these pairs, like "farther/further" and "nauseous/nauseated," are treated as officially distinct by traditional style guides but are increasingly used interchangeably in everyday speech. Formal writing, such as academic or professional documents, is the context where sticking to the traditional distinction still matters most.
Memory tricks that actually hold up
The best mnemonics link the spelling to the meaning. The e in stationery goes with envelope, the a in stationary goes with at rest. The two t's in discrete are separated by an e, just as discrete things are separate. The pal in principal suggests a friend, such as the head of a school. Make up your own if these do not stick, since a personal picture is easier to recall. For pairs with no clean trick, such as compose and comprise, one reliable approach is to write the correct sentence in a notebook and review it a few times, instead of trying to re-derive the rule every time.
How to use a dictionary for this
A good dictionary lists the part of speech and a sample sentence, which is often all you need to settle a confusion. Many include a usage note that explains whether a distinction is firm or relaxing. Look at what part of speech you need in your sentence first: a noun, a verb, or an adjective. For example, advice and advise differ by part of speech, so naming the part of speech settles which spelling is needed. Style guides like the Chicago Manual of Style, the AP Stylebook, and Fowler's usage guide are also useful, but they sometimes disagree, so follow the one your reader or publisher uses.
Frequently Asked Questions
Is it wrong to say "I feel nauseous" instead of "I feel nauseated"?
By the traditional rule, "nauseated" is the technically precise choice, but "nauseous" is now so widely used to mean "feeling sick" in everyday speech that most people will understand it immediately either way.
What is a fast way to remember "stationary" vs. "stationery"?
Stationery, the paper kind, has an "e," the same letter that starts "envelope" and "letter" β both things associated with writing paper.
Are "farther" and "further" really different, or is that outdated?
The traditional distinction (physical distance vs. figurative extent) is still taught in most style guides, but many everyday speakers and even some publications now use the two words interchangeably, especially for "further."
What is the quickest way to separate "e.g." from "i.e."?
"E.g." introduces examples (for example), and "i.e." restates or clarifies (that is). If you are listing some items out of many, use "e.g." If you are naming the one thing you mean, use "i.e."
Is it "affect" or "effect" in most sentences?
Most often, affect is the verb and effect is the noun. If you can put "the" or "an" before it, you probably need effect.
Are British and American spellings of these pairs the same?
Mostly, but some differ. In British English, "practice" is the noun and "practise" is the verb, while American English uses "practice" for both. "Licence" and "license" follow the same pattern in British usage.