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I like to visualise things to try and understand them as an idea not a rehearsed rule. I like to think of hazu in terms of it being a notch in a bow for the arrow as I hear the meaning stemmed from there. I won't get into that too much but...

Is there any way to understand べき in a similar manner? I hear it is related to "CAN" because it uses the same kanji "可き". How should I understand its nuance?

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語源ってそんなに重要?語源を知ったところでその単語や表現を能動的に会話や文章の中で使えるようにはならないと思うんだけど。語源を研究するのは上級者になってからでい‌​いのでは?つまり、これくらいの日本語が自由に読めない学習者なら語源の知識なんて不必要だと思う。ほかにあるでしょう、今やんなきゃいけないことが? – l'électeur Nov 3 '13 at 11:50
@TokyoNagoya I think what to learn is everyone's own choice. If etymology is really useless for learning a particular expression (and sometimes it is useless, although often it can be helpful), then any motivated student would realize that learning about such etymology didn't help him/her much in deepening his/her understanding of the (modern) language. In that sense, I think that any reasonable question about the language deserves an answer, no matter who asked. Asking the right kind of questions is after all part of the learning process. – Earthliŋ Nov 3 '13 at 12:25
That said, reasonableness of the question could be improved. @Nathan What do you mean by "visualize" the etymology? Do you expect any etymology to be pictorial or do you want just an easy-to-remember picture-to-word association? – Earthliŋ Nov 3 '13 at 12:31
Since the grammar structure is "Xをするべきだ", where べき is a noun being modififed by Xをする... then it makes sense that I should know what that noun is... I want to know what I am modifyig. It doesn't need to be pictorial, I just need to know what it a べき is, or what べき means? Is that such a moronic pursuit Tokyo Nagoya? – Nathan Nov 3 '13 at 12:44
It's not an noun. べき is etymologically an adjective and is sui generis in modern Japanese. – Zhen Lin Nov 5 '13 at 8:29

Is there any way to understand べき in a similar manner?

I am afraid not.

べき is the 連体形 form of inflectable suffix べし in Classical Japanese, and in Classical Japanese, 連体形 form was used as an abstract noun by itself in general. According to Daijisen, the widely accepted origin of べし is adverb うべし in Classical Japanese, which meant something similar to “indeed.” I do not know the origin of うべし.

But I doubt that this knowledge (plus the knowledge about the origin of うべし) helps you understand the meaning and usage of べき in the modern Japanese. While I respect your experience that etymology helped you understand the meaning and usage of some words, I think that you were lucky in those cases.

how should I understand its [べき’s] nuance?

I do not know. Maybe you can check the examples of べき and べし in a textbook or a dictionary?

Just in case, do not try to remember the meanings listed in a dictionary. We use words in English (e.g. “should”) without remembering its ten or so meanings listed in a dictionary. You should do the same thing with words in Japanese.

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Was the 連体形 used as abstract noun in Classical Japanese in more generality than in Modern Japanese (e.g. 締切り, やりとり, 申し込み, etc.)? – Earthliŋ Nov 3 '13 at 14:54
@Earthling: You are talking about the noun derived from the 連用形 (not 連体形) form in Modern Japanese. Thinking about it, it was quite inaccurate to say that 連体形 was used as an abstract noun in Classical Japanese. The construct such as ~べきなり should be understood as zero-nominalization rather than べき being a noun. I will try to update the answer, but it will take some time. (In addition, my motivation to make it accurate is kinda low because the point of this answer is to show that etymology does not always help the understanding in usage.) – Tsuyoshi Ito Nov 3 '13 at 15:42
@Earthling べき does seem fairly unusual for a 連体形 (as you point out with the example of べきだ). There's an interesting write-up of this word and its history on Matt's blog, no-sword.jp, which touches on this point. – snailplane Nov 3 '13 at 17:14
@Earthling I thought べし was a 形容詞? Instead of becoming べき->べい it became べき->べきだ. – user54609 Nov 5 '13 at 0:35
@Earthling, the 連体形 べき is also irregular, since it isn't べい. – dainichi Nov 6 '13 at 15:47

The best way to understand べし is to look at how it was used in classical Japanese, because some of that usage extends into modern Japanese. Then, separately look at how it is used commonly in modern Japanese.

Etymologically, べし probably came from the adjective うべし as Tsuyoshi Ito said, just shortened for use as an auxiliary adjective, and additional meanings/usages were added to it over time.

べし attaches to the 終止形{しゅうしけい} or 連体形{れんたいけい}(ラ変). In modern Japanese, the distinction between 終止形 and 連体形 has deteriorated, but in classical Japanese, for instance, this would mean 食{た}ぶべし rather than 食{た}ぶるべし, すべし instead of するべし, あるべし instead of ありべし. - As a given rule there is always an "u" sound before the べし.

The classical conjugation for べし is like all classical ク形容詞{けいようし}, (べから・べく、べかり・べし・べき、べかる・べけれ、べかれ).

However in modern Japanese べき basically transformed into a restricted な adjective. Or, if you don't want to call it that, a 終止形 form べき(だ)、and 連体形 form for use with particles べき(なの), that are like the corresponding な conjugations were added to the list of possible べき forms in modern Japanese. Other strange and restricted な adjectives that used to be 形容詞 include 同{おな}じ (mix of original and な-adj-ish form) and 大{おほ}し→大{おお}きな (連体形 only).

These conjugations mix together in modern Japanese, using the classical for phrases like ~べからず, ~べからざる, すべき - and modern for するべき, ~べきだ.

Anyway, the best way to understand is studying on your own. 頑張ってください。

Knowing the etymology is very helpful to understanding sometimes, but you have to remember that it isn't always everything.

You could think of しかし in terms of of しかじ (classical form), which is just a conjugation of the verb 如{し}く, which is the basis for a whole slew of other things, and give yourself a headache, or you could just pay attention to how and where it is used regularly in sentences.

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It does feel a little weird calling べき a な-adjective without any additional explanation, since there's generally no な when it's placed before a noun. (The only べき+な I can think of off the top of my head is in べきなの.) I think I would compare べき to 同じ to illustrate its exceptional usage. – snailplane Nov 28 '13 at 13:27
@snailboat 同じ is a great example, as it is also a シク形容詞 and な adjective. I forget when I read the な-adj argument, but べき fits the bill for a な adjective, with the exception that it does not use な/連体形 (which makes calling it one a bit of a stretch) but I think it is like how classical adjectives do not use かる for 連体形 but their original form, if that makes any sense. I'll edit the answer. – Kafka Fuura Nov 28 '13 at 15:35

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