how to know when to use each?
一月 (いちがつ) means January.
一ヶ月 (いっかげつ) means a period of one month.
I don’t know what the usage of 一月 (ひとつき) is.
The usage for months isn’t the same as days where, for example, 五日 means both a five day period and the fifth day of the month.
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The examples on jisho for ひとつき are in context of doing something once a month. So it’s basically “monthly” or in some contexts “per month”.
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Oh, @nanobite beat me to it.
Yeah, Jisho definition is “one month,” sentence examples show a usage of “x times/month, in a month,” sort of thing, mostly: ひとつき #sentences - Jisho.org
「 ひとつき 」 is the same as 「 いっかげつ 」, but isn’t used in print much because it’s visually indistinguishable from 「 いちがつ 」.
I’d recommend using [Kotonoha] for a much wider range of example sentences to pull from.
[Note: Upon first use, scroll to the bottom and click on the ‘Agree to terms’ button:]
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Can’t you use 月に一回 to mean “once a month?” I don’t think you have to use 一月に一回
Yes, that’s correct. 月に一回 is more common.