An error survives because it never stops you being understood. Nobody corrects it, you cannot hear yourself make it, and after a few hundred repetitions it is not a mistake any more, it is simply the way you say that sentence.
The name for a form that has settled like that is fossilisation, and the useful part of the term is what it says about cause. A fossilised error is not a gap in your knowledge. You can very often state the rule it breaks, correct it in somebody else's writing, and produce it wrongly again ten seconds later, because what has hardened is a habit and not an idea.
Eight errors that survive into advanced English
- I am agree is wrong. Agree is a verb, so it is I agree. The error comes from Romance languages, where the equivalent uses to be.
- I am 25, not I have 25 years. English uses be for age, and the have version is the direct translation from French, Spanish and Portuguese.
- people are, not people is. People is already plural and takes a plural verb.
- information is uncountable, and so are advice, furniture, news and equipment. There is no an information and no informations.
- he doesn't, not he don't. The third person s survives in the negative and in questions, and losing it there is the most audible grammar error there is.
- explain it to me, not explain me it. Explain, suggest and describe cannot take a person as their first object.
- I look forward to hearing, not to hear. Here to is a preposition rather than part of an infinitive, so an ing form follows.
- its and it's. The first is possessive and the second is it is. Native speakers get this wrong at least as often as learners.
Correction stops arriving the moment you become comprehensible
Early on, everybody repairs you. Teachers repair you because that is the lesson, and strangers repair you because they have to in order to work out what you meant. Then you cross the line where you are reliably understood, and both sources switch off within about a month of each other. A stranger who understands you has no reason to interrupt, and doing it anyway would be rude.
That is the stall, and its shape is unusual: nothing gets worse, nothing feels stuck, and the machinery that had been building your grammar quietly stops. Whatever is unrepaired at that moment is what you keep. The learner who spends the following decade adding vocabulary is not wrong to do it, and it will not touch any of this, because vocabulary is a thing you acquire and these are things nobody is telling you about.
Why the ones that last are the ones you cannot hear
You monitor your own speech for meaning rather than for form. What reaches your attention is anything that threatened to make you misunderstood, which is why a word you could not find or a sound you know you mangle both register immediately, and register as failures.
A dropped article changes nothing about what you meant, so it never reaches the monitor at all. Neither does a third person s, which is unstressed and swallowed at speed and carries information the sentence has already given twice over. Neither does a preposition imported from your first language, which feels correct from the inside because it is correct in the language that supplied it. These are not harder to fix than the audible ones. They are harder to notice, which is a different problem and needs a different method.
The checker catches the written half
LanguageTool is the practical tool here: free in the browser with an extension, with a paid tier and a limit on how much text one check will take, and it flags missing articles, subject and verb disagreement and a good share of imported prepositions. It replaced the spelling and grammar checker built into a word processor, which caught spelling reliably and almost nothing else, and which trained a generation to believe a clean document meant a correct one.
Be exact about its reach. It reads writing, and most of what this page is about happens in speech, where nothing flags anything. What it can do is give you an inventory: run a month of your own emails through it, ignore the one-off slips, and write down the three things it says every single time. Those three are yours, and knowing which three you have is most of the work.
Unfreezing one, which is the only rate that works
Take one item off that list and make it expensive on purpose for a fortnight. Write it where you will see it. Check for that one thing alone, in writing first, slowly, before it goes anywhere. Do not check for anything else at the same time.
One at a time is not modesty about effort. Under speaking speed you get roughly one thing you can consciously watch, and a learner monitoring for four errors catches none of them and speaks worse while trying. The point of the fortnight is to reintroduce the cost that disappeared when people stopped correcting you, and a self-imposed cost only works if it is aimed at one target.
The two that appear on almost every list are a, an, the, or nothing at all, and how to tell which and in, on, at, and the ones no rule explains, both under the rules that decide what a sentence is allowed to mean. Neither is difficult. Both are quiet.
What follows is a question about who is going to tell you. A correction only does anything when it lands inside an attempt you were already making, which means you need somebody willing to interrupt a person they can already understand, and that is a thing to arrange rather than to hope for: finding someone to speak to, and what to do in the hour. If nothing at all seems to be moving, the wider version of the same question is the stalls every learner hits, and the way through each.