Confusion about し＋なさい ("do + do"?)


I've just been thinking that if verb + なさい constructions are a combination of
verb stem + なさる's 命令形, how on earth is something like しなさい possible. they both
mean "do", don't they ("do do!) perhaps I'm completely over-thinking this. Is
there a reason as to why なさい can't be used its own?




The suffix なさい does not function like the verb なさる ("to do").

Syntactically:

  * the suffix なさい attaches to the 連用形 of verbs
  * the verb なさる behaves essentially exactly like する: it is either is on its own, or attaches to the root of any suru-verb.

Semantically:

  * the suffix なさい makes the verb into an imperative and provides no honorification (it almost provides the opposite by indicating the addressee is in a position lower than the speaker)
  * the verb なさる means "to do" and provides subject honorification.

So to me, the なさい in しなさい is unrelated to "to do", it simply converts する into
an imperative. (If you don't buy that a form of なさる can perform as a purely
grammatical element, consider what function the "do" in "I do not know" is
performing.)

Regarding the construction 漢語名詞＋なさい (e.g., 結婚なさい), [it is
productive](http://home.alc.co.jp/db/owa/jpn_npa?sn=184), and according to
@TokyoNagoyaさん, it seems to have a different, slightly softer connotation than
the 〜しなさい construction. I am not sure where exactly it falls on politeness
spectrum of the various different imperatives
(〜しろ、〜して、〜してくれ、〜してください、〜しなさい、〜なさい, etc.）.

