Every-so-often Grammar/Phrase/Word

毎に (goto ni)
Basic meaning: every
Comments: something takes place regularly in succession after a certain temporal or spatial interval
Key sentences:

1. 私は三時間毎に薬を飲んだ。
   watasi wa sanjikan goto ni kusuri o nonda.
   I took medicine every three hours.
2. この電車は二駅毎に止る。
   kono densya wa futaeki goto ni tomaru.
   This train stops at every other station.
3. 正月は家毎に角松を立てる。
   shougatu wa ie goto ni kadomatu o tateru.
   Pine tree decorations are put up at every house on New Year's day.
   See footnote 1.

Formation: {quantifier,noun} 毎に
Additional examples:

1. 一課毎に試験がある。
   ikka goto ni siken ga aru.
   There is a test after each lesson.
2. 木村さんは会う人毎に挨拶している。
   kimura-san wa au hito goto ni aisatu site iru.
   Mr. Kimura is greeting everyone he meets.
   See footnote 2.
3. 学期毎に先生が変る。
   gakki goto ni sensei ga kawaru.
   The teachers change every semester.

Usage notes:

1. This form is most commonly used with quantifiers, such as in key sentences 1 and 2. Using this form with nouns, such as in key sentence 3, is possible, but rare. One acceptable form, as in example sentence 1, is when the noun involved has some related temporal interval (as in 一課毎に (ikka goto ni) (= after each lesson), as the lessons likely take place on a schedule). For other types of nouns (人 (hito), 家 (ie), etc), you should consider using どの~も (dono ~ mo) (= every ~), みんな (minna) (for people), ずつ (= each, respectively), 毎- (mai-) (= each), etc.

Footnotes:

1. While grammatically correct, this sentence is not very natural (see the usage notes). More natural is:
   正月はどの家も角松を立てる。
   shougatu wa dono ie mo kadomatu o tateru.
   Pine tree decorations are put up at every house on New Year's day.
2. More natural is the following (using みんなに (minna ni) instead of 毎に (goto ni)):
   木村さんは会う人みんなに挨拶している。
   kimura-san wa au hito minna ni aisatu site iru.
   Mr. Kimura is greeting everyone he meets.