I have recently read a sentence in a story .
The four of you may be proficient, each in your own subject.
I wonder why is the comma used before each. Is the word each adverb or pronoun in this sentence ? Looking forward to your wisdom replies ---
In this case "each" is used as a pronoun and is the subject of the dependent clause. Edwin is correct that the comma is used because the clause is a parenthetical and requires punctuation to offset it from the rest of the sentence. It doesn't have to be a comma. The sentence could also have been written:
[...] proficient-- each in your own subject.
[...] proficient (each in your own subject).
If the sentence continued after the word subject, another punctuation mark would be used to indicate the end of the parenthetical:
[...] proficient, each in your own subject, [additional content]
If I were writing a sentence like this I would use a comma rather than a different punctuation mark because the comma suggests a pause in speaking while other punctuation suggests other things (like a hasty additional comment with dashes or a quiet aside with parentheses).