>>2
Are you talking about the rules I wrote in the Japanese Ask Questions thread 2? Here it is and I made some corrections too.
Here is the pattern:
When two 音読み(on-yomi) kanji combine to become a 熟語(jyukugo), if the kana of the first kanji ends with つ(e.g.発~はつ 実~じつ 說~せつ 必~ひつ) or ち(e.g. 一~いち, 日~にち), the first kanji becomes 促音(sokuon) according to the rules as follows.
Rule No.1: when combine with the second kanji that begins with kana belongs to kana columns that have no 濁音(dakuon), which means あ(a) な(na) ま(ma) や(ya) ら(ra) わ(wa) six columns, the first kanji does NOT become 促音(sokuon).
e.g. 説明(せつめい),発明(はつめい), the kana め of 明(めい) belongs to ま行(ma column) and ま行(ma column) has no 濁音(dakuon), therefore 説(せつ) and 発(はつ) do not become 促音(sokuon).
Rule No.2: If the second kanji begins with kana belongs to kana columns that have 濁音(dakuon), which means か(ka) さ(sa) た(ta) は(ha) four columns,
A>> If the kana of the second kanji begins with 濁音(dakuon), the first kanji ALSO does NOT become 促音(sokuon).
e.g. 発動(はつどう), 実在(じつざい), the kana ど of 動(どう) and ざ of 在(ざい) are 濁音(dakuon) therefore 発(はつ) and 実(じつ) do not become 促音(sokuon).
B>> If the kana of the second kanji begins with 清音(seion) that has 濁音(dakuon), the first kanji MUST become 促音(sokuon).
e.g. 実験(じっけん), 実行(じっこう), the kana けof 験(けん) and こof 行(こう) belong to か行清音(seion of ka column) and か行(ka column) has 濁音(dakuon) therefore 実(じつ) MUST become 促音(sokuon) and pronounce じっ(sit).
Rule No.3: When the second kanji begins with one of は ひ ふ へ ほ (seion of ha column) five kana, follows rule no.2B>> PLUS the first kana of the second kanji (は or ひ or ふ or へ or ほ as mentioned) MUST become 半濁音(handakuon).
e.g. 脱皮(だっぴ), 出発(しゅっぱつ), the ひ of 皮(ひ) and は of 発(はつ) belong to は行清音(seion of ha column) therefore follow rule No.2B>> that 脱(だつ) and 出(しゅつ) become 促音(sokuon) PLUS 皮(ひ) and 発(はつ) MUST become ぴ(pi) and ぱつ(patsu).
Sorry for my clumsy writing but the point is to understand the pattern, not to learn the rules by rote.