I know use onyomi and kunyomi for kanjis but i confused for use other means of kanjis for example 日 this kanji means sun,day Kunyomi:ひ,び,か Onyomi:にち,じつ This example 昨日 means さくじつ why isn't use にち for kunyomi reading日曜日(にちようび) why is use び for this word we can use ひor か. I don't understand for Kunyomi or onyomi reading for kanjies
1 Answer
First of all, you'd most commonly read 昨日 as きのう (a special reading), though the reading さくじつ is used more formally.
Secondly, you have to bear in mind that the reading of a character is determined by what surrounds it (i.e. the word it is being used in). You can't predictably tell which reading will be used in a given word if the kanji in that word have multiple readings; you just have to remember what they are for a given word.
naruto's comment on your question contains a great answer.