dir or dich?
when we say thanks, we use dir "ich danke dir". As i know, i thank you is indicates direct object and Akkusative. so Akkusative personal pronoun is "dich" why cant we say "Ich danke dich" ? why dativ personal pronoun "dir" is coming here?
Please clarify me
There are some verbs that are always followed by dative. Danken is one of them.
Ich danke dir.
Ich folge dir. (I follow you)
"Danken" is an intransitive verb, i.e. a verb that has no direct object to modify. In this case, it is literally "I thank (to) you," which is the exact equivalent in meaning to the theoretical though highly inconvenient and almost-certain-to-never-happen "Ich gebe dir Dank," like giving thanks.
Indeed, the definition for "danken" on the German Wörterbuch means to show (someone) thanks, so ultimately, it is intransitive. There are other words with the same meaning to our English counterparts that are structured unlike how we would in English, but as with any language, it can be mastered. I hope this helps.
P.S. I just read Rams.1's message that said that the comments were already helpful and therefore thanked the commenters, but I felt the urge to provide an answer that I believe had a very good explanation and clarification.
You're thinking too much from the perspective of English. In German, danken is a verb that requires the dative, as if you were to say "my thanks goes to you". In English, "you" is a direct object with "I thank" - although it might be good to keep in mind that the difference between the dative and accusative case has disappeared in English, so originally "thank" might very well have worked in a similar way as the German form, I don't know to be honest.
Thinking from the perspective of German rather than English - writing a German sentence rather than translating an English one - takes time, but you'll need it to understand these little details that can't be simply explained in English.
The Dative follows danken. The cases of nouns always depend on the verbs (and prepositions).