Why most grammar tools quietly rewrite your voice
You paste a paragraph, the checker underlines a comma splice, and the replacement it offers has also merged two sentences and swapped “because” for “as”. The flag was real, so you accept it. Three sentences later the paragraph is fluent, correct and no longer quite sounds like you.
The mechanism is bundling. A single accept button carries the genuine correction and a style preference at once, and there is no way to take the first without the second. Across a 900-word statement of purpose that is perhaps forty clicks, each one defensible on its own.
So the two are kept apart here. A missing article, a subject that disagrees with its verb, a full stop sitting outside the quotation mark — all corrected. A sentence that is merely long, a passive you chose on purpose, a word a thesaurus would rank higher — left exactly where it is.
What this tool refuses to call a mistake
Prepone is a word. It has a precise meaning, no one-word equivalent, and an entry in the Oxford English Dictionary — and most checkers still underline it in red. Lakh and crore are the units your reader in Pune counts in, and rewriting ₹4.2 lakh as 420,000 helps nobody who has to read it.
Revert meaning reply, doing the needful, out of station — each is correct inside the register it belongs to, and an office email in Chennai is that register. Whether you want revert in a covering letter to a firm in London is an editorial decision, not a grammar one.
Those forms pass. What does not pass is the genuine slip standing next to them — “she is having two brothers” where the tense is wrong, “discuss about” where the verb already carries its preposition, “one of my friend” where the noun needs a plural. The line sits between usage and rule.
Where a perfect sentence says the wrong thing
The tool reads the sentence, not your intention. Write that the deadline is the 14th when you meant the 4th and every word is spelt right, the tense agrees, the preposition fits. Nothing gets flagged. Nothing is wrong with the sentence — only with the fact sitting inside it.
The same holds for a supervisor's name spelt consistently wrong, ₹40,000 typed where you meant ₹4,00,000, and a leave application dated a month out. Each of those is clean English. A grammar checker of any kind is the wrong instrument for them, and a clean pass often makes them harder to spot.
It also will not tell you whether an argument holds, whether a paragraph belongs where you put it, or whether the essay answers the question that was asked. Those judgements stay with you. Read the corrected text once more for meaning, slowly, because that pass catches what this one cannot.