I thought the Phrase 'so that' needs no comma. Why one sentence has a comma, whereas the other one doesn't?

1. First print two copies of each set of cards and then paste them on a cardboard backing so that students can't see thorugh them.

2. Entries are made whenever a natural pause in the reading occurs, so that the flow is not interrupted constantly.

Can we think of 'so that' as the word 'because'?
Stictly speaking, a comma is not necessary.
Can we think of 'so that' as the word 'because'? No, as 'so that' indicates result and 'because', reason.
in such a way that
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So, the example provided before, with a comma is totally "incorrect" according to traditional grammar rules? I could not find the answer in a paper grammar book yet.

I don’t know about it’s being incorrect, but it’s unnecessary for sure.
I comma should not be used in either sentence because the clause that follows the conjunction is not an independent clause that can stand alone as a complete sentence. A comma should only precede a conjunction if an INDEPENDENT clause follows. Neither "that students can't see thorugh them" nor " that the flow is not interrupted constantly" are independent clauses/complete sentences. See example one on the excellent Purdue Online Writing Lab (OWL): http://owl.english.purdue.edu/owl/resource/607/02 / Also, Georgia Southern University has a web page that explains how to punctuate dependent and independent clauses. Perhaps you will find it as useful as I have. http://class.georgiasouthern.edu/writingc/handouts/punctuatingdependent.htm
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Hi -

Rules in grammar aren't quite as set in stone as many people seem to think they are. As a high school English teacher, I clearly remember disagreeing about a punctuation issue with one of my colleagues. I proved my point by showing her an example from the grammar textbook I was using at the time. She then proved her point (entirely contradictory to mine) by showing me an example from the grammar textbook she was using.

This link should answer your question very clearly -


I believe the first example of so that that you submitted is entirely correct as it is.

Commas can be used to help clarify written ideas when necessary, which may be why a comma was included in the second example you submitted. However, I personally do not think any additional clarification is needed in that fairly simple sentence. I would say it is incorrect to include a comma there.

And yes, so that is a subordinating conjunction, just like the word because.

Hope this helps!
Strictly speaking, it does need a comma. Removing "that" leaves "so", which does not change the sentence's meaning. The word "so" is a coordinating conjunction (like the words and/but), and when a coordinating conjunction is used to connect two independent clauses (students can't see through them ), a comma needs to precede the word.