One of the greatest frustrations that new language learners may have with Japanese is the seeming incomprehensibility of the way that Japanese kanji or counters can be modified. We’ve already discussed one aspect of this in Rendaku, but there are other rules as well.
Let’s use the “minute” counter as an example. You will likely be told to memorize this (I’ll use romaji for now)
1 minute = ippun
2 minutes = nifun
3 minutes = sanpun
4 minutes = yonpun or yonbun
5 minutes = gofun
6 minutes = roppun
7 minutes = nanafun
8 minutes = happun
9 minutes = kyuufun
10 minutes = jyuupun
This is due to a rule called “handaku”. I almost promise you’ll never be taught this in any kind of beginning classes.
I’ll link to sites with more detail, but the general rule is this: If a syllable ends with a nasal sound, a chi sound, or a tsu sound, then it’s replaced with a pp. So sanbun becomes sanpun, hachibun becomes happun, etc.
As you can see from even what I wrote, this isn’t entirely regular, and some memorization is required. But maybe this will at least take some of the mystery out of what you’re otherwise told to memorize. See this page for more details, and we’ll add more as we discover it ourselves.