When to use shichi, nana

• 
  miso January 23, 2014 at 9:51pm

  That 's exception rules. The only way to put them into your mind.
  
  four men
  4人 (yo-nin) ※No one pronouces "4人" as "shi-nin"
  4個 (yon-ko) ※No one pronouces "4個" as "shi-ko"
  
  nine
  9個(kyu-ko) ※No one pronouces "9個" as "kyu-ko"
  
  We may pronounce "9人" as both "kyu-nin" and "ku-nin".
  
• 
  mog86uk January 24, 2014 at 10:52am

  It seems, especially with counter words, that yon/nana/kyuu are used for many more situations than shi/shichi/ku.
  
  One of the main places I notice that shi, shichi, and ku need to be used is with some parts of time and date:
  
  ~ 4/7/9th month (-gatsu) = shi-gatsu (April), shichi-gatsu (July), ku-gatsu (September)
  
  ~ 17/19/27/29th day (-nichi/-ka) = (ni-)juu-shichi-nichi, (ni-)juu-ku-nichi
  
  ~ 7/9th Hour of the day (-ji) = shichi-ji (7 o'clock), ku-ji (9 o'clock)
  
  shi/shichi/ku are the normal versions for some counters -- maybe because their sound when used with the counter clashes with similar sounding words? or their sound just fits better?
  
  ~ example: 7人 = 'shichi-nin' -- maybe this is because 'nana-nin' would sound a bit more weird? (I'm just guessing here!) ^^
  
  Also, yon/nana/kyuu seem to be what is used to make 40/70/90, 400/700/900, 4000/7000/9000. And it seems they are what is normally used for numbers above ten, for the ending number 4/7/9, too.
• 
  mog86uk January 24, 2014 at 11:33am

  I've got a feeling that 'shichi-juu' is actually used a lot for 'seventy', so I'm not sure whether 'nana-juu' is more common. There's kind of a stigma for using 'shi' and 'ku' though, so 'yon' and 'kyuu' are much more clear-cut favourites.
  
  Also, I guess a problem with me thinking that 'nananin' might not be used because it could sound weird, is that the Japanese choose to use 'shichi-ji' rather than 'nana-ji'! :P