There are a number of words that have more than one reading in Japanese. Sometimes, as shown by this webpage, it’s a matter of the reading changing over time. For example, I read somewhere that 「世論」 is supposed to be read as 「よろん」 but so many people misread it as 「せろん」 that it eventually emerged as an alternative reading. For words like this, choosing a reading is merely a matter of preference and depends on the popularity of the reading since the meaning is the same. However, some words have different readings and different meanings to go with them. We’ll look at two that I can think of right now (「頭」 and 「家」) and how you would identify the correct reading.
１．頭が痛いから早く家に帰りたい。
-I want to go home because my head hurts.２．彼の家には14歳を頭に3人の子供がいる。
-In his house, there are 3 children, 14 being the oldest.
In the first sentence, 「頭」 is talking about the speaker’s head (the thing on your neck) and so we should read it as 「あたま」. However, in the second sentence, we are talking about the 14 year-old being at the head of the rest of the children. When we are using 「頭」 to mean “head” as in “chief”, or “the first”, we read it as 「かしら」. Amazingly, the English word for “head” also contains both meanings (though we don’t change the reading).
Finally, 「家」 can have two readings depending on whether the speaker is talking about his or her home or just a generic house owned by anybody. Your own home is read as 「うち」 which probably has something to do with 「内」（うち） meaning inside. The reading for a generic house is 「いえ」 (not to be confused with 「いいえ」).
So in order to figure out when to use which reading, 1) learn the difference in meaning, and then 2) look at the context of the sentence. So can you identify the correct readings for 「家」 in the examples sentences?
It makes me sad to see how improvements made in the Japanese language have been thrown away so easily.
The word うち means “inside” and has the derived meaning of “home”. Originally, it was assigned as kun-yomi to kanji 中 when used to mean “inside”, 家 when used to mean “home”, and to its own kanji 内 as well.
Trouble was, 中 and 家 already had other readings なか and いえ respectively with the same meanings as うち. This lead to ambiguity. So the Japanes (the Education Department, I think) decided to clean up the language: 内 was うち, 中 was なか and 家 was いえ. How sensible!
It was part of other reforms too, like 明日 only used for みょうにち, while あした and あす being written in hiragana. Using 明日 for あした became frowned on. Again, it sensibly avoids ambiguity–occasions arise where you don’t know which reading the author intended. My 旺文社 dictionary dates from that period and follows these guidelines.
In the last 20 years or so, however, I’ve noticed that the Japanese have gone back on these reforms. I was perplexed why Japanese were using such formal language as 明日, when it dawned on me, to my horror, that they were using it for あした again.
While I am fond of some of these usages, I feel this is a retrograde step. I guess the Japanese too were too fond of them to lose them, so good sense gave way to feeling good. Oh well, I guess we have to live with it.
In Japanese language courses I’ve taken (using the Genki text) no one ever said anything funny about 明日. Interesting to learn that there were changes in convention.