Why these particular mistakes are so common
Nearly every entry on this list involves homophones — words that sound identical when spoken but are spelled differently and mean different things. Because they sound correct out loud, they slip past a writer's ear, and because they are spelled correctly as standalone words, they slip past a spell-checker too.
The fastest way to catch them yourself
The substitution test used throughout this list — mentally expanding a contraction back to its full words — catches the majority of these errors in seconds. Reading a sentence aloud also helps, since your ear will often notice an odd rhythm even when your eye skips right past a typo.
When a "rule" is actually more flexible than it sounds
Some traditionally taught rules, like never ending a sentence with a preposition or never splitting an infinitive, are treated as much more flexible by modern style guides when the alternative sounds stiff or unnatural. The mixups above are different — they change the literal meaning of the sentence, which is why they are worth getting right even in casual writing.
A reliable proofreading routine
First, finish the draft and leave it alone for a short time, since you read what you meant to write when the text is fresh. Then read it aloud or with text-to-speech, which exposes missing words and odd rhythm. Next, search the document for your personal weak spots. If you tend to mix up "its" and "it's," search for the apostrophe and test each one by expanding the contraction. Finally, change the font or print the page, because the unfamiliar view helps the eye catch errors. A grammar checker is useful as a second pair of eyes, but it makes mistakes, so treat its suggestions as questions rather than commands.
Rules, usage, and the difference between them
Some points in this list are firm: "could of" is simply wrong, and "your" and "you're" are different words. Others are matters of formality and style. Guides disagree about whether "fewer" and "less" must be separated in every case, or whether a comma is needed in certain places. Dictionaries record how people actually use the language, while style guides recommend a standard for a particular audience. When you write for school, work, or publication, follow the guide your reader expects. When you write casually, clear communication matters more than strict adherence.
American and British conventions
Differences between American and British English, such as color and colour, or the placement of punctuation relative to closing quotation marks, are conventions and not errors. What matters is staying consistent within a document. The mix-ups on this page are different because they are not about regional preference. They change the meaning of the sentence for readers in every English-speaking country.
Frequently Asked Questions
Does spell-check catch any of these?
Rarely. Spell-check flags misspelled words, but "there" and "their" are both spelled correctly — the mistake is a word-choice error, not a spelling error, so basic spell-check tools miss it entirely.
Is "less" always technically wrong for countable nouns?
By the traditional rule, yes, though the distinction is loosening in casual speech. Formal or professional writing is still safer sticking to fewer for countable items.
Is ending a sentence with a preposition actually incorrect?
No — most modern style guides consider it perfectly acceptable when the alternative phrasing sounds awkward or overly formal.
What is the single fastest way to catch these before hitting send?
Read the sentence back and mentally expand any contraction to its full two words — if "it is," "you are," or "they are" still makes sense in that spot, the apostrophe version is correct.
Is it wrong to start a sentence with "and" or "but"?
No. Starting with a conjunction is accepted in most modern style guides and is common in good writing, provided the sentence is complete and the use does not become a habit.
Which is correct, "between you and I" or "between you and me"?
"Between you and me" is the standard form, because a preposition such as "between" takes the object form "me." The version with "I" is often an overcorrection by speakers who were once told not to say "me and him."
Should I use "who" or "whom" in everyday conversation?
In casual speech, "who" is used for both in most contexts, and "whom" feels formal. In formal writing, it is safer to use "whom" for objects of verbs and prepositions, using the he or him test described above.