There is an old saying in computing that programmers have been muttering to each other since the 1950s.
Garbage in, garbage out.
The idea is simple. If you feed a computer bad data, you get bad results. It does not matter how powerful the machine is, or how sophisticated the software. If the input is junk, the output will be junk. The computer processes whatever you give it faithfully and without judgment. Give it garbage, and it will produce beautifully processed garbage.
Your brain works the same way with English.
The English you speak is the English you fed yourself
The English that comes out of you, your speaking, your writing, your instinct for what sounds natural, is shaped directly by the English that goes into you. Your brain builds its model of English from whatever input you provide. Feed it rich, natural, sophisticated, real English, and it builds a rich, natural, sophisticated model. Feed it simplified, artificial, flattened textbook English, and it builds a simplified, artificial, flattened model.
The machine does not judge the input. It just processes it. So the quality of what you put in determines the quality of what comes out.
What garbage input looks like
“Maria goes to the shop. She buys two apples and a bottle of milk. The shop is near the park. Maria likes the park.”
That is a textbook dialogue. It exists in thousands of English learning materials around the world, and it is, linguistically speaking, dead on arrival.
Nobody talks like that. Nobody writes like that. The sentences are artificially simple. The vocabulary is deliberately limited. The rhythm is flat. There is no personality, no nuance, no voice. It is language stripped of everything that makes language alive.
And yet millions of English learners spend hundreds of hours consuming exactly this kind of input, because their textbook provides it, their course prescribes it, or their app generates it.
Now compare that to this, from the opening of a podcast you might actually enjoy listening to.
“So I was walking home last night, absolutely knackered, and I ran into this guy I hadn’t seen in about fifteen years. And the weird thing is, he looked exactly the same. Like, exactly. I honestly thought I was hallucinating.”
Same language. Same English. But this is alive. It has rhythm, personality, humour, colloquial vocabulary, natural phrasing, connected speech. It is what English actually sounds like when real people use it for real purposes.
Which of these two inputs do you want your brain building its model from?
Why this actually matters
The textbook sentences are technically correct. The grammar is accurate. So what is the actual harm?
The harm is in what is missing.
When your brain builds its model of English from textbook input, it builds a textbook model. Sentences always short, clear, and complete. Vocabulary limited to the most common, most neutral, most colourless words. No contractions, no fillers, no slang, no personality.
Then you walk into a real English conversation and discover that nobody speaks like your textbook. Sentences are long, messy, sometimes unfinished. People say gonna and wanna and kinda. They interrupt each other and abandon sentences halfway through.
Your brain, trained on textbook input, has no model for any of this. It encounters real English and freezes, because real English looks nothing like what it was trained on. This is garbage in, garbage out. Not because textbook English is wrong, but because it is impoverished, and a brain trained on lifeless English produces lifeless output.
The learner who has consumed thousands of hours of real English, podcasts, books, shows, conversations, has a completely different model. Their brain has absorbed the messiness, the informality, the real rhythms of how English actually works in the wild. When they speak, something natural comes out, not because they are more talented, but because they were fed better input.
The Duolingo problem
Let’s talk specifically about Duolingo, because it is the most widely used language app in the world and it is, from an input quality perspective, a genuine problem.
Duolingo is very good at what it does best, getting people to open an app every day. Full marks for engagement. But look at the actual English input you receive during a session.
“The elephant drinks milk.” “My grandmother is fast.” “The bear eats bread at the airport.”
These are real sentences people spend real time on. There is no context, no conversation, no situation in which a human being would ever need to say that a bear eats bread at the airport. Your brain processes these faithfully. It absorbs the vocabulary, registers the grammar patterns. But what is it actually building? A model of English disconnected from how the language is used by actual people in actual life.
Compare that to thirty minutes spent listening to a podcast where two people have a genuine conversation about something interesting. The vocabulary is natural. The grammar is natural. The rhythm, the fillers, the hesitations, the humour, all of it is natural. Same thirty minutes. Radically different input quality.
The grammar exercise trap
Grammar exercises have the same problem, and in some ways it is worse, because they feel more productive than they are.
Fill in the blank with the correct form of the verb. She went to the cinema yesterday. You write went. You get a tick. You feel like you learned something.
But what did your brain actually process? A disembodied sentence fragment, no context, no story, no reason to care whether she went to the cinema at all. The grammar point was practised in isolation, and grammar acquired in isolation tends to stay in isolation, because it was never connected to real communication in the first place.
Now imagine encountering the same grammar point in a novel you are reading on LingQ instead.
“She went to the cinema alone that night. She hadn’t planned to. But the apartment was too quiet after he left, and she couldn’t stand the silence, so she grabbed her coat and walked to the old Odeon on the corner, the one that still had velvet seats and sold popcorn in paper bags.”
The grammar is identical. She went. Same tense, same construction. But look at the context, the emotional weight, the vocabulary surrounding it. Your brain is not just processing a grammar point. It is processing a moment, an experience, a rich piece of English encoding dozens of vocabulary items and stylistic features simultaneously.
One of these inputs produces a brain that can fill in blanks on a worksheet. The other produces a brain that can understand and eventually produce real, living English.
A necessary caveat, beginner content has its place
Before anyone accuses me of telling beginners to go straight to Shakespeare, let me be clear.
At the very beginning of your English journey, when your vocabulary is close to zero and real content is genuinely incomprehensible, there is a necessary period where simplified content is appropriate. Beginner courses, comprehensible input channels, graded readers, content designed for learners with controlled vocabulary and clear, slow speech. These are the training wheels that get you moving.
This is not garbage input. This is appropriate input for your current level. The idea is content that is mostly comprehensible with a little stretch beyond it, and at the beginner level that means simple content.
The problem is not using beginner content when you are a beginner. The problem is still using it once you are not a beginner anymore, still doing Duolingo exercises and grammar workbooks at an intermediate level when you are more than capable of engaging with real English that would build a far richer model of the language. The training wheels were useful. But at some point you have to take them off.
What quality input actually looks like
It is real. Written or spoken by real English speakers for real communicative purposes, not manufactured or sanitised for learners.
It is interesting to you specifically. A podcast about a topic you are genuinely curious about, a book in a genre you love, a show that keeps you watching one more episode. Content that pulls you in because it is compelling, not because someone told you it was good for your English.
It is varied. Different speakers, different styles, different registers, formal and informal, British and American and Australian. The wider the variety, the richer the model your brain builds.
It is slightly challenging. Not overwhelming, but not easy either. Content where you understand most of it and the remainder is new territory your brain is working to decode.
It is language you would actually want to use. The English in a compelling podcast is English you might find yourself using in conversation. The English in the bear eats bread at the airport is not.
The upgrade
If you are still spending time on Duolingo, grammar workbooks, or textbook exercises, and your English level is intermediate or above, redirect that time toward real content.
Replace the Duolingo session with thirty minutes of a podcast you love on LingQ. Replace the grammar workbook with a chapter of a novel. Replace the textbook exercise with an episode of a show on Lingopie. Same time investment, dramatically better input, and over weeks and months, a dramatically better model of English being built inside your brain.
The consistency you have already built, the daily habit, the regular contact with English, is the valuable part. Keep it. Just change what you are feeding the machine.
You are what you read, and listen to, and watch
There is a version of this idea that applies to food. You are what you eat. Feed your body junk and it performs like junk. Feed it quality nutrition and it performs at its best.
Your English is what you input. Feed your brain flat, artificial sentences and your English will sound flat and artificial. Feed it podcasts where smart people discuss fascinating ideas, novels where skilled writers craft beautiful sentences, conversations with real people about things that matter, and your English starts to sound like that too. Natural. Alive. Full of the rhythm and personality that only comes from real English produced by real people for real purposes.
If you want to test that in real conversation, you can find a teacher on iTalki, or book a session directly with me, and put the better input straight to work.
Try This
Look honestly at what you have been feeding your English this week. If most of it has been graded exercises, drills, or app based sentences, and your level is past true beginner, swap just one session this week for something real. A podcast, an article, a show, on any topic you actually care about.
Notice the difference in how it feels, and trust that your brain notices the difference too.
If you have a question about your English, something you have been stuck on or curious about, let me know in the comments. I read every one, and I might turn your question into a future post.
If this post helped, a like lets me know and a restack helps it reach another learner who might need it.
Richard, English fluency coach in Auckland, New Zealand.
Tools I recommend:
LingQ — feed your brain real English through reading and listening, with every word tracked and every encounter building depth
Lingopie — real TV shows and films with natural dialogue and interactive subtitles
Glossika — structured sentence repetition that trains your mouth on well formed, natural English, available in both British and American English
StoryLearning — compelling, story driven English listening at just the right level
iTalki — find a conversation partner whose English is genuinely worth absorbing, or book a session directly with me
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I think it's also important that English teachers feed themselves on rich and interesting English input so that they may provide their students with the best model.