A spellchecker catches strings that are not English words. What gets marked on your writing is almost always a real English word sitting in the wrong place: their for there, its for it's, affect for effect, principal for principle. Every one of those passes a checker without a flicker.
So this is a short list rather than a skill, and the list is roughly the same for everybody. Learn the forty and the subject closes, which is unusual and is worth taking advantage of.
Eight facts, four of which are one rule
- i before e except after c is unreliable and has more exceptions than it has uses. It is worth knowing only so you can stop trusting it.
- colour and color, British and American, and the same drop applies to favour, honour and neighbour.
- organise and organize. The z form is standard in America and also correct in Britain, where Oxford uses it, so this is a choice rather than an error.
- travelled and traveled. Britain doubles the l before an ending, America does not, and the same applies to cancelled and modelling.
- centre and center, with the same swap in theatre, litre and metre.
- double the consonant before -ing after a short stressed vowel: run gives running, begin gives beginning, and open gives opening because the stress is on the first syllable.
- -ible and -able. Able is far commoner and is used with complete words; ible usually follows a root that is not a word on its own.
- silent letters are historical. The k in knee and the gh in night were once pronounced, which is a fact about the past rather than a rule you can apply.
English spelling is more regular than its reputation
The irregularity is concentrated, not spread. It sits almost entirely in the few hundred commonest short words, the ones that came down from Old English and were written before the spelling settled: one, through, enough, said, women. You already know all of those, because you met them in your first fortnight.
Long words are mostly regular, because they arrived later from Latin, Greek and French with their spelling attached and never got worn down. That is the opposite of what learners expect, and it is a relief: the frightening polysyllabic word in an academic paper is usually spelled exactly as it looks, and the four-letter word is the one that is lying to you.
Three patterns worth an hour
The doubling rule before a suffix is the one with real coverage. A single consonant doubles when the syllable before it is stressed and does not when it is not, which is why it is stopping and beginning but visiting and offering. Once you can hear the stress, you can spell the whole family.
The British and American differences are systematic rather than random, which means they are four rules and not a list: the u in colour, the ending of centre, the doubled l in travelling, and -ise against -ize, with the last one messier than the others because both are used in Britain. Pick one convention and be consistent, because a document that mixes them is more noticeable than one that picks the wrong one.
And -able against -ible, which is genuinely arbitrary in the middle but has a usable edge: if the part before it is a complete English word, it is nearly always -able. Comfortable, readable, reliable. Where the stem is not a word on its own, -ible is more likely.
Where it stops: trusting the underline
The stall is outsourcing. A writer stops attending to spelling entirely because software handles it, which is a reasonable decision for the class of error the software handles and leaves the other class completely unattended.
What makes it expensive is that the unattended class is the one a reader notices. A misspelling that a checker would have caught reads as a typing slip. A homophone in the wrong place reads as somebody who does not know the difference, which is a judgement about the writer rather than about the keyboard, and it is the judgement people actually make.
Nothing corrects it either, because the page comes back clean. Years can pass with a writer confident that spelling is solved, and the same six words are being got wrong in every document.
Making the machine carry your own forty
The autocorrect list in a word processor takes entries you add yourself, which is the feature almost nobody uses. Put your own repeated errors in it and they are repaired as you type, and the list itself becomes the record of which words you personally get wrong, in one place rather than scattered across three years of marked documents.
It replaced a list of your own misspellings on a sticky note, which nobody consults while typing and which is therefore a list of things you will get wrong again. Free, in every word processor.
The limitation is real and worth deciding about deliberately. Autocorrect fixes the symptom silently, so you stop learning the word, which is fine for twenty words you will never write by hand and is not fine before a handwritten exam. If there is a paper exam ahead, keep the list and do not let the machine repair it for you.
What it looks like when it goes wrong
The recognisable failure is a document with no spelling errors and two homophones in it, returned with both circled. The writer's reaction is usually surprise rather than recognition, because from the inside the document had been checked.
The second is a mixed convention: colour in the first paragraph, color in the fourth, usually because part of the text was drafted somewhere with a different default. It is trivially fixable and it looks careless in a way that a consistent choice never does.
The doubling rule above depends on hearing where the stress falls, which is which syllable to hit, and how the wrong one changes the word, and the other class of thing that gets marked without being an error of vocabulary is commas, semicolons and apostrophes, settled. Spelling and punctuation are the two finite subjects inside emails, essays and reports that read like a native wrote them, and the only two that can be finished.
What remains after both of those are closed is the class of fault nothing marks at all, because none of it is wrong: the sentence that runs too long, the paragraph with no claim in it, the register a degree off. That is where the writing actually gets decided, and it begins at length, rhythm, and where a sentence breaks.