Hello
@WonkoTheSane さん,
Thank you for your reply. Your reply caused me to check, and actually it was me who was plain wrong, namely I was wrong in what the actual verb forms are (see below for details). I joined this list to learn Japanese, but it turns out that what I'm actually learning is English

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Often, the people who would say "To hear him speak English" are the type of people who might also use the word 'were' specifically because it feels right to the phrasing.
I looked things up, and found this webpage that explains the subjunctive in the English language in a clear and accessible way:
Subjunctive | Grammar | EnglishClub
Which states that for the 3rd person singular (he), the subjunctive forms of "to be" are:
present subjunctive : he be
past subjunctive : he were
So the phrase
"you would think he WERE an American"
is actually completely correct grammatically. The grammar is the correct, traditional, formal "British English" grammar, and the language style is somewhat formal (or maybe better: the language style is not informal). The "were" is correct because it is a subjunctive form (past subjunctive, 3rd person singular) of the verb "to be".
At the end of the webpage I mentioned, there is a table comparing formal and informal language usage. The left of the table (formal) shows the "were" form, and right of the table (informal) shows the "was" form that I used in my previous email. So in actual usage of the language, I think that one may say that in practice there are *two forms* of the English subjunctive, namely a non-informal one (using the correct and tradtional form "were") and an informal one (using "was").
I think that I now also understand how I came to believe that "was" was the correct subunctive form: when learning English, I've always tried to acquire a relatively "modern" style of English (as a software developer and software documentation writer, this seemed to be the right thing). So it seems that in the subjunctive thing, I have overcorrected, and had somehow so gotten used to using "was" that I forgot the theory and came to believe that the non-formal "was" was the actual correct form of the subjunctive.
It makes sense to me when you say that "To hear him speak English" lies in the same area of formal/informal language use as the "traditional" subjunctive form "were", i.e. that the "were" fits right in with the beginning part of the sentence.
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Another interesting point in your reply is the variation in American English.
American English is extremely varied, which is why questions like this don't have one answer. In my home we were specifically taught not to ask "Can I have the..." when requesting food at the table to be passed, instead saying "May I have..." or "Please pass the..." which is different from many people. In the south, especially among African Americans, it is often taught not to say "I'm sorry" and instead to say "I apologize" because the first can imply a negative about the speaker instead of a specific response to a situation.
Between the extreme variations of American English dialect, and the way many people will fit a word choice based on the style of phrase, I respectfully submit that it is an entirely reasonable phrase.
That is very interesting -- それ は 大変 面白い です.
I get the impression that the variation in English is bigger than that in Dutch (my native language) and than that in German (where I've been living and working now for 12 years). Dutch is a small language which naturally brings a type of "restrictiveness", but German with 90 million or so speakers is not a small language and has e.g. quite large vocabulary. Still, my impression is that in German, the "rules" that are observed by native speakers (even in informal speach) for what is correct and not correct are clearer ("more restrictive", thus allowing less variation) than they are in the English language.
I'm taking this up this "extensively" because I'm wondering how this variation aspect is in the Japanese language. -- Is Japanese more like English in that "everything goes" (i.e. the "rules" allow great variation in language usage), or is it more like German in that the rules are clearer thus allowing less variation ? I'm not referring here to the well-known formal/informal/polite registers in Japanese (because each of these registers seems to have its own set of clear rules). Instead, I'm wondering about the sitation inside of each (or any one) of the formality registers. I.e. inside the normal "polite" (丁寧語) register (using です), how "clear" and rigid are the rules for what is correct and incorrect language, i.e. what is the "bandwidth" allowed by the rules ? Inside 丁寧語 language, is there a situation similar to English, in that language constructions or word forms that are in some ways "incorrect" grammatically (or that are incorrect in certain older forms of grammar) are nevertheless very widely used ?
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With best regards,
Menno ( メンノー )