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这个是从古英语过来的,OED的词源学词典里解释的是
(a) In the idiomatic I,we,you,he, etc.had better, the original construction was me,us, etc.were betere(orbet) = it would be more advantageous for me
这个短语以前的形态是
宾格+were betere(orbet)
The dative pronoun was subsequently changed into the nominative,I,we,were better[…].
Finally, this was [abandoned] for the current I had better= I should have/hold it better, to do
随着语言的发展,宾格变化为主格,were betete形式变化为had better。
you had better not do something you had better do.something.