There are a lot of word’s letters that can’t be pronounced, like The D in Fidgety, the K in know, and the H in hour.
Is there a rule or something that I most follow to know it.
As with much of English spelling, there are patterns (I hesitate to call them rules), but they are always partial, with various exceptions.
The "d" in "dge" is in a way silent; but unlike the other cases you mention, it is not completely silent, because it is there in the pronunciation /ʤ/ - but the same sound is sometimes written without the "d", as "ge".
There is a pattern here, but it is not obvious.
You know that often a single consonant letter shows that the vowel before it is "long" (as it's traditionally called, but generally it means a diphthong)? So in words like "tapping" and "taping", the double "p" indicates the "short" vowel /æ/ and the single "p" the "long" vowel /ɛɪ/.
The spelling "dg(e)" is often used like a double letter in the same way. So the word "cage" has a "long" vowel /kɛɪʤ/, while "cadge" is short /kæʤ/.
A similar thing happens with "ch" versus "tch", though there aren't very many examples of words where a single vowel before "ch" is pronounced "long". But note that words which already have "long" vowels and diphthongs before the consonant are usually spelt with "ch" (eg "screech", "beach", "couch", "pooch"); whereas words with "short" vowels are usually spelt "tch": "stretch", "batch", "notch".
As with all rules of English spelling, there are exceptions; but this works in many cases.