Can you conclude that just by my writing the question?
closed as not constructive by onomatomaniak, Robusto, MετάEd, KitFox♦ Apr 20 '13 at 15:00
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Non-native speakers of any language struggle with the flow of the adopted language. There are subtleties of usage which can be taught, but which are hard to learn precisely, and usually come from growing up with the language, learning it by gradual osmosis, until everything about the language is as natural to the native speaker as breathing.
For just a moment, let's consider the (rather unusual, admittedly) analogy of breathing. A "native breather" breathes air with ease, with consistent regular movements of the rib cage, and without ANY thought. But what if you had to be TAUGHT to breathe? It would be tricky to learn to move your chest in just the right way. And while you're still trying to develop the ability to move your rib cage in a natural, UNTHINKING way, you might be able to breathe effectively, but any "native breather" would immediately recognize that you are expending an unusual amount of effort and thought to accomplish the simple task of breathing, which would identify you as a "non-native" breather.
So, language has flow, movement, dynamics, fluctuations in intensity, complicated word choices, subtle gradations of appropriateness, and much MUCH more. All of this is learned by native speakers through DECADES of trial and error, absorption from other native speakers, and endless practice, until their use of their native language is easy, fluid, confident, free of hesitation, and fundamentally characterized by intuitive, instinctual use of the language. All of this is summed up in a single word: "fluency." Non-native speakers struggle with the subtle dynamics of a language, such that they are not usually fluent, and this is immediately detected by native speakers, who know the language as well as they know their own names.
Let's look now at your original question: "When I write any sentence in English every native reader can tell I am Europen, how?"
A native speaker would not say "any sentence"; we would say "a sentence." This is a very good example of the kind of mistake made by non-native speakers; they have some uncertainty in the use of the language, and so they feel compelled to use a word that explicitly emphasizes the concept they have in mind (you wrote "any," because you are concerned to emphasize that this problem happens no matter what you write) rather than a word that is more fluent, and less emphatic (in this case the word would be "a"); native speakers are confident that they will be understood, so they are comfortable choosing a word that is commonly accepted to mean "any," without overly emphasizing the concept.
"In English" is formal, and again not the usual way of expressing things; we tend to use the adjective prior to the noun, thus: "English sentence." We find it more definitive and efficient. "In English" is unduly formal, and so it sticks out as unusual, and not the sort of thing that a native speaker would say in casual, common speech.
"Native reader" is another clue, because it is artificial. We almost never use this phrase; we would say "native speaker" every time, in any context. You, on the other hand, as a non-native speaker, once again respond to your own internal demand to make sure that your usage is correct. You feel it is necessary to use "native reader," because you are not confident that "native speaker," which you think doesn't fit with the context of writing, will be understood by most readers. But a native speaker is perfectly happy NOT to change the commonly used "native speaker," even when referring to writing, because the native speaker realizes that it will be perfectly well understood to say "native speaker." The native speaker realizes that this term is effective and standard, no matter what the context.
I could go on: the choice of "can tell" is awkward and non-standard, the choice of "every" instead of "any" (very subtly off the mark), the comma splice at the end (which actually reflects a flow of speech that is not usual when utilizing spoken English), and so on. Furthermore, native speakers have not only an ease with the language, they also use the language in creative and almost unpredictable ways, to make things more interesting, and to color what they say; they want people to pay attention to what they say, and they want to be understood. They especially dislike sounding too formal, because it puts people off.
So there are MANY ways your example sentence could be written, and you already have good suggestions for revisions. Let me give you a much more colorful, casual example, just so you can see how relaxed native speakers are in their use of the language:
This is only one of MANY alternatives. You are limited only by your own imagination.
Native speakers interpret a text as having been written by a non-native speaker in two ways. The first and most obvious sign is that the text contains a grammatical error, often betraying interference from the writer's own language.
For example, the German native speaker who writes:
Or the Russian who writes:
The second sign is that the text contains errors of usage. Usage can either be defined as "single-word grammar" or more broadly and nebulously as "the way native speakers construct their utterances."
An example of usage as a "single word grammar mistake" is a question currently on the ELL sister-site:
In English we can talk about, complain about, ask about, but not say about.
Your own question is an example of the second type of usage problem. Although it is perfectly correct grammatically, it contains very subtle clues that you are not a native-speaker. It would take more time than I have to write a complete analysis of why, but Andrew Leach has provided a native-speaker version, and my own would read:
As Andrew indicated, the "how" is incorrectly placed in your example sentence. It should (or could) be:
how come native speakers can always tell I'm from Europe when I've only written a single sentence?
how is that native speakers can tell I'm European from every single sentence I write?
how can native speakers always clock me as European when I've barely written a single sentence? [clock: colloquial]
England is in Europe, of course, although we often feel separate from "Europe", so Europe can stand for the European Continent here.