Sentence Correction

ruplun wrote:The book Mechant of Venice, a classical series,which is written by ...

In the SC we find a comma is required after series
But in "the moment which is lost is lost forever" we are not using any
comma. Can anyone explain this?

Using commas with relative pronouns, (who, what, where, which*, that*, ...) can definitely be challenging. It might help to quickly review a more general comma rule (and then see how this applies).

Major Comma Uses:
(1) Separate items in a list (X, Y, and Z)
(2) Used with words like but, and, or, for, nor to connect 2 independent clauses (if we don't have the connector word, we must use a semi-colon - something that is far more typical on the GMAT)
(3) after introductory (lead-in) modifyinf phrases: Known for his brilliant works of fiction, Charles Dickens...
(4) Offset Non-essential Words, Phrases and Clauses

Use #4 is the one we want to look at for the which question. There are actually 3 major types of these non-essential sentence elements:
(a) interrupters (however, thus, of course, ...)
(b) appositives (re-namers: "Lisa Anderson, the vice president at my company, arrived late to the party")
(c) non-essential adjective phrases and clauses (if they are essential, we do not use a comma)

So, what do we mean by essential versus non-essential - How can we tell? Essential adjective clauses are necessary to understand and define the subject. Take a look at the following examples:

Bob Jones, who fled the scene yesterday afternoon, was apprehended today.

The suspect who fled the scene yesterday afternoon was apprehended today.

Notice that in the first sentence, dropping the who clause does not impact my understanding of the action: Bob Jones was apprehended today. I know who was arrested! But in the second example, I needed the who clause because "The suspect was apprehended today" leaves me unsure of who was nabbed!

Which vs. That Distinction
Here is the one tricky rule.

(a) that = use for essential clauses = no commas
(b) which = use for non-essential clauses = with commas (technically you can use which with essential clauses but it is far less common)

The movie that topped box office sales this week is showing at 9:30pm.
The new Disney movie, which topped box office sales this week, is showing at 9:30pm.

To be non-essential, I need to be able to nail down the subject of the sentence without that information. Note that ALL adjective phrases/clauses are providing additional information about their subject - the question is more about how much info we need to adequately specify the subject in the world.

I hope this helps!!

Whit