Looking for some clarification on what seems like a simple grammar point.
Some of the example sentences for に (Frequency) use ‘間（かん）/か’ to indicate the time interval, but others don’t. I tried looking at Genki, and see the same inconsistency.
Is there a rule for when to include 間（かん）/か and when you can leave it out? Does it matter if you say “一週に” instead of “一週間に”? “一年間に” instead of “一年に”?
It would also help if every time frame wasn’t 一.
Hey 
So, this will gonna be a fairly long post so get ready for a ride.
There are two ways to use this grammar:
First one is to use counter only, without number (but it only works as “per one period of time”, basically per week, per month but never per 2 months, per 2 weeks, per 3 weeks, per 3 months etc):
If so you can use:
週に〇回
月に〇回
もし植物に水あげてくれるなら、週に２回お願いします。
月に１回きちゃんに電話をかけます。
父は年に２回出張します。
すくなくとも日に６時間は寝る必要がある。
父は日に２回タバコを吸いに行く。
Ohh, and if you want to use grammar this way then:
もし植物に水あげてくれるなら、週間に２回お願いします。is not good
ケ月に１回家族に電話をかけます。is also no good
週間 by itself (without a number) is mostly used as a noun like in:
週間の天気予報 = 週間天気予報 (weather forecast for a week)
Another way is to use a counter of a span of time WITH a number.
And here things get crazy.
Basically: you use 間 in 時間 (hours)、週間(weeks)
もし植物に水あげてくれるなら、1週間に２回お願いします。
父はすくなくとも１時間に１回はタバコを吸いに行く。
It is optional for days (BUT it has to be more than one day, so you cannot use 1日間, but ２日間, 3日間 etc are OK) and for years.
父は一年間に２出張します。 OK
父は一年に２出張します。OK
父は一日に２回タバコを吸いに行く。OK
父は一日間に２回タバコを吸いに行く。NOT OK
父はすくなくとも2日間に２回タバコを吸いに行く。OK
父はすくなくとも2日に２回タバコを吸いに行く。OK
(Though you can say: 父は1日の間に２回タバコを吸いに行く。)
It is not used with seconds, minutes, months.
Good idea
Great breakdown, thanks! While I’m not the original Poster, I certainly learnt something here.
The usage of 間 could be its own grammar point, huh? 