I'm a little shy to ask this question, because I feel like I'm supposed to know this by now, but I never really understood the difference between 年 and 年間 when indicating a duration of time. When I first came to Japan, I always said 年間 (as in, １年間ぐらい日本にいます or something), but I would usually hear people around me say 年 without the 間. Could someone clarify this for me?
Good question, not really sure to be honest. I've never heard 何年間日本にいるんですか？ from a Japanese person. Looking at ALC's example sentences I don't see any examples of 年間 and いる. It seems to be more marking something that happened over that time or encompassed that period. There are a ton of examples of ～年間にわたって for example.
This is just a guess (I'll ask, if I remember), but I wouldn't be surprised if 年 deals with length, while 年間 deals with a period during which something occurred.
Sorry to dig this post up after it was laid to rest 5+ years ago but I was wondering something similar. I was wondering when you would use 間 in general. I understand how to use it when talking about あいだ but as かん I am still confused. On a different site the question came up as in days. For example 一日 is いちにち or ついたち depending on if you are talking about a period of one day or the first. But if you are talking about more than one day in a period would you use かん as in 十日間 or leave it as 十日. The reason I ask is because when you talk about a period of 2~3 days it in 二三日 but 十四日から二十日まで is used for from the 14th to the 20th. How would you then say for 14 to 20 days I did something or am planning to do something. I guess to tie it back to the original post how would you also use it for time as I have heard 五年後 but never 五年間前 when talking about past or future.
so the suffix 間（かん） refers to a duration of time, whereas without the suffix, it's just a point in time. So 一時 = one o'clock, 一時間 = one hour. So for your question about 14-20 days, you can say 十四日間から二十日間.
