generally speaking, i and u turns into ĭ and ŭ (we're using these symbols to mean "inaudible, or almost inaudible, unless pronounced oddly clearly or being otherwise very stressed") after:

f, h, k, sh, ts thus:

いち ichi - ichĭ ("ich") ろく roku - rokŭ ("rok") わたくし watakushi - watakŭshĭ ("wataksh") たくさん takusan - takŭsan ("taksan") です desu - desŭ ("des") つき tsuki - tsŭki ("tski") ちくしょう chikushoo - chĭkŭshoo "chkshoo" for this reason we have some odd things when counting various objects, or otherwise in some compound words. notice how it's very easy to see why and where this happens, if we simply ignore that ŭ exists: rokŭ ka = rokka ろっか hyakŭ ka = hyakka ひゃっか On the letters that can have " (nigori - muddled): if these letters begin the second word in a compound (as in the h in "dog-house"), they'll become their " forms. For example: ちぢしじふぶひびかがさざつづただ In compounds of words that come from Chinese, f and h can change to p (°). This is most easily noticed in numbers because Chinese numbers are usually used when counting. F and h also ALWAYS change to either b or p when coming after ん. In some Japanese-origin words, "Tokyo people" change h into p because they like to double consonants in general, but since h itself can't be doubled it needs to turn into p instead. another explanation is that sometimes it might be from ancient times when h was pronounced as p: さん ふん = さんぷん じゅん ふ じゅんぷ yahari = yappari yohodo = yoppodo " = nigori ° = han-nigori because it's considered harsh to have too many nigori letters in one word, if a word beginning in one is at the front of a compound then the second part of the compound doesn't get it. thus kaza + kami = kazakami, not kazagami. overall, which words actually get these changes can sometimes in fact just depend on the speaker. thus oo + saka = both oozaka and oosaka. regarding numbers: chi disappears entirely in front of most consonants. obviously, it'd be difficult to say "ichka" or "hachsai" for example, that's probably why: ichĭ ka = ikka hachĭ sai = hassai however there is another possible explanation for this. hi tends to turn into shi or sh, especially in vulgar tokyo: hige = shige hĭto = shto last u in juu disappears before almost every consonant ku in roku and hyaku disappears (since it's not said to begin with): rokŭ ka = rokka hyakŭ ka = hyakka other changes simply follow the normal nigori and han-nigori rules. thus: rokŭ hyaku = roppyaku (h changes to p because it's a chinese compound) ichĭ fun = ippun (ch disappears, also f changes to p in chinese compounds) many words with initial " come from chinese, ex. dozou = a godown go = august zash(i)ki - a room exceptions: de (particle) = from nite dore = idzure