I find it useful to distinguish bet

I find it useful to distinguish between "that" and "which." If the subordinate clause helps to
define the noun, that is, is essential and could not be removed, then it is appropriate to use
"that" and to use no commas to separate the clause. Often the word "that" serves as the direct
object of the subordinate clause. In this case, the word "that" may often be discarded. If the
subordinate clause adds extra information of a non-defining nature, that is, is parenthetical
and could be removed, it is appropriate to use "which" and to separate the subordinate clause
with commas.