Why some Kanjis when I study them can be read in some different ways, but sometimes when I study them along with another kanji, they are read in a completely different way? How can I know how to read them?

For example:

I studied 「人」 to be read as 「ひと」、「-り」、 「-と」、「ジン」and 「ニン」. But in 「大人」 is not read in any of this ways, instead it is read as 「-な」.

Why is it like this?

Thank you!

質問を翻訳

Well, there’s a small mistake in what you said because in 大人 the character 人 is not equal to “na” just as the character 大 is not “oto”.

The pronunciation of those two characters together is “otona” just because that is the word for “adult”, not because of the kanji that represent it. This means that the choice of kanji only reflects the idea of “big”+”person”=“adult”, but that is not related to the way it is pronounced.

So basically there are some words like this one that have a completely different pronunciation from its kanji, and there’s no other way but to learn them by heart.

Examples:
大和 - yamato (and not “daiwa”)
足袋 - tabi (and not “ashibukuro”)

For what concerns all the other regular words, you will just have to learn which is the correct reading of the kanji from the ones you have studied, but if you keep studying eventually you will be able to guess which is the correct one :)

Examples:
一人 - hitori
人口 - jinkou
三人 - sannin
人々 - hitobito