How Our Spelling & Grammar Checker Works

Most spelling and grammar tools are black boxes — you paste text in, corrections come out, and you have no idea how it happened or how much to trust it. This post explains exactly how the SmartWriteTools Spelling Report and Grammar Checker work, what they can and cannot catch, and why we built them the way we did.

Two layers, not one

The toolkit uses two separate, complementary approaches to catch writing errors — intentionally kept apart so results are easy to understand and trust.

Layer 1 — Browser spellcheck: the red underlines you see as you type
Layer 2 — The Spelling Report and Grammar Checker tools: our own pattern-matching layer, run on demand

Keeping them separate was a deliberate decision. Mixing them into one result list makes it harder to know what changed and why. Transparency builds trust.

Layer 1: Browser spellcheck

The textarea in the grammar checker has a single HTML attribute added to it: spellcheck="true". That is the entirety of layer one.

Your browser — Chrome, Firefox, Safari, or Edge — maintains its own dictionary and uses it to underline words it does not recognise in red. Right-clicking an underlined word gives you a list of suggested corrections. This system is fast, reliable, handles names and jargon gracefully (it learns from your previous corrections), and works entirely on your device. No text leaves your browser.

The main limitation is that browser spellcheck — unlike a dedicated grammar checker — does not understand context. It will not flag "their" when you meant "there", because "their" is spelled correctly — it is just the wrong word.

Layer 2: The Spelling Report

The Spelling Report tool catches what browser spellcheck misses: words that are spelled a consistent wrong way that many people use — and context-blind errors like "alot" where the mistake is not a single word but a missing space.

How it works: a lookup list

The tool holds a curated list of 75 of the most commonly misspelled English words, each paired with its correct spelling. When you run the report, it scans your text for every word on that list using a regular expression — a precise pattern-matching technique — and reports any it finds.

Example from the list

Pattern: \brecieve\b

The \b markers are word boundaries — they ensure "recieve" is only matched as a standalone word, not as part of a longer string like "unrecievable". The match is case-insensitive, so "Recieve", "RECIEVE", and "recieve" are all caught.

When a match is found, the tool reports it as:

Output format

❌ recieve → receive 2×

❌ alot → a lot 1×

The occurrence count (2×, 1×) tells you how often the mistake appears, so you know whether it is a one-off typo or a consistent habit to correct.

What the Spelling Report does NOT do

It is important to be clear about the limits. The Spelling Report:

Does not catch every possible misspelling — only the 75 words on its list
Does not use a full dictionary lookup (that is what browser spellcheck is for)
Does not understand context — it cannot tell if "bare" should be "bear" in a given sentence
Does not send your text anywhere — the entire check runs in your browser

Think of the Spelling Report as a focused second opinion, not a complete spell checker. Used alongside browser spellcheck, the two layers together cover a broad range of common mistakes.

Layer 2: The Grammar Checker

Grammar checking is significantly harder than spell checking. A grammar checker needs to understand context, not just spelling. The word "your" is not wrong — "your going to the shop" is wrong. Catching that requires understanding that "going" is a verb following a pronoun, which should trigger "you're" (you are) rather than "your" (possessive).

Full grammar checker engines — like those in Grammarly or Microsoft Word — use large language models trained on millions of sentences. We are not building that. Instead, the Grammar Checker targets a small, specific set of high-confidence rules where the pattern is reliable enough to flag without generating false positives.

The rules and why we chose them

Rule 1 — your / you're

Pattern: "your" followed immediately by a verb ending in -ing (going, doing, being, etc.). In natural English, "your going" is almost always a mistake — the correct form is "you're going" (you are going). The list of -ing verbs is explicit and curated, keeping false positives close to zero.

Rule 2 — there / their / they're

Pattern: "there" or "their" followed by an -ing verb. "There going" and "their going" are both almost certainly "they're going". Same logic as rule 1 — the -ing verb is the reliable signal.

Rule 3 — its / it's

Pattern: "its" followed by certain words that imply the verb "to be" (going, clear, possible, important, not, a/an). "Its going to be fine" is almost certainly "It's going to be fine". The follow-on word list is conservative to avoid flagging legitimate uses of the possessive "its".

Rule 4 — capitalisation after a full stop

Pattern: a sentence-ending punctuation mark (. ! ?) followed by a space and then a lowercase letter. This is reliable because English always capitalises the first word of a new sentence. The only edge cases are abbreviations (e.g., "Dr. smith") — which is why the tool says "review in context" rather than auto-correcting.

Rule 5 — double spaces

Pattern: two or more consecutive space characters. Double spaces are almost always unintentional — a relic of typing habits or copy-paste from other sources. This is one of the most reliable rules in the set.

Rule 6 — should/could/would of

Pattern: "should of", "could of", "would of". The correct forms are "should have", "could have", "would have". This mistake is extremely common in informal writing and the rule has essentially no false positives — "should of" is never correct in standard English.

Rule 7 — less / fewer

Pattern: "less" followed by a plural noun (ending in -s). The rule of thumb: use "fewer" for things you can count (fewer words, fewer mistakes) and "less" for uncountable quantities (less time, less water). "Less mistakes" should be "fewer mistakes". This rule is conservative — it only flags clear cases with a following plural.

What the Grammar Checker does NOT do

The grammar checker does not:

Catch subject-verb agreement errors ("the team are" vs "the team is")
Detect tense consistency issues
Flag dangling modifiers or misplaced clauses
Understand sentence structure beyond pattern matching
Use AI or machine learning of any kind

Each of those problems requires deep language understanding that goes well beyond what a pattern-matching system can reliably do. Adding rules that generate false positives would make the tool feel untrustworthy — which is worse than not catching the error at all.

Why no AI?

A fair question. AI-powered grammar tools — trained on billions of sentences — can catch subtler errors, understand tone and register, and suggest rewrites. So why not use one?

Three reasons:

Privacy. Sending your text to an AI service means your writing leaves your device. For personal, commercial, or sensitive content, that is a real concern. Everything in SmartWriteTools runs locally.
Transparency. When an AI flags something, it often cannot explain why. When our tool flags "your going", it shows you exactly which rule triggered and why. You can agree or disagree based on reasoning, not a confidence score.
Reliability. AI suggestions can be confidently wrong. A curated ruleset with conservative thresholds produces fewer false positives — and in a writing tool, a false positive (flagging correct text as wrong) damages trust more than a missed error.

The two tools working together

The recommended workflow is straightforward:

Write or paste your text into the editor
Glance at the red underlines from browser spellcheck and right-click to fix obvious typos
Run the Spelling Report to catch habitual misspellings in the curated list
Run the Grammar Checker to flag the specific confusables and punctuation issues it targets
Review each suggestion in context before accepting it — both tools say "review in context" for a reason

This combination catches a wide range of common writing mistakes quickly, without requiring an account, an internet connection beyond the initial page load, or sending your text anywhere.

Choosing the right grammar checker for your needs

Not every grammar checker is right for every situation. Here is a quick guide to matching the tool to the task.

If you are writing personal or confidential content — drafts, private notes, business documents — use a grammar checker that runs locally in your browser. The SmartWriteTools grammar checker never sends your text to a server. Nothing leaves your device.

If you are writing long-form professional content and want deeper suggestions — tone, style, word choice — an AI-powered grammar checker like Grammarly or LanguageTool may be worth the subscription. Be aware that your text is sent to their servers and used to improve their models.

If you are a student or blogger checking for common mistakes before publishing, a free grammar checker with a focused ruleset is usually enough. Most student writing errors fall into a small number of categories — your/you're, capitalisation, double spaces, should have — that a rule-based grammar checker handles reliably.

Whatever grammar checker you use, always review suggestions in context before accepting them. A grammar checker is a second opinion, not a final editor.

Verifying your corrections

After running the spelling and grammar checker and making corrections, a text compare tool provides a useful final check. Paste the uncorrected draft into Panel A and the corrected version into Panel B. The word-level diff shows every change — useful for confirming that only the flagged errors were corrected and no unintended edits crept in.

Try it on your own text

Paste anything into the editor and run the Spelling Report and Grammar Checker — both are free, instant, and private.

Open SmartWriteTools →