when do we use a comma after "which"?

Your question is confusing...commas usually come before which...so I will answer the question I think you are trying to ask.

You use a comma when "which" introduces a relative or nonessential clause (they both mean the same thing.)

A nonessential clause is one that adds extra information to the sentence, but is not essential to the main point of the sentence.

"The answers, which were given in Latin, were very impressive." That they were given in Latin is not the main point, but extra information. The fact that the answers were good is the main point.

A rule of thumb is "which" introduces nonessential clauses, and "that" introduces essential clauses, but that is just a general rule - it doesn't mean the one should go around putting commas in front of all "which"s.

Comma Before Which

After Which

Those are two things which, when added together, make something else.

There's an example.

all the time, otherwise you use "that". so you can say

i love my clock that stands in the hall OR

i love my clock, which stands in the hall

Orphans. Beck all the mothereffing way.

This is a question which, in my opinion, illustrates the decline in the standard of English as taught in the UK.

i eat food, which i buy from the store.