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Sometimes I have difficulty clearly seeing the causal realtionship as to decide properly on the issue of whether to put a comma before the word 'so' or not. Help. I think the 1st sentence shows a clear causal relationship but not the second and the third sentences. Do you have some tips I can follow to develop an eye to see clearly?

1. He saved money so he can pay for his college education.

2. His parents are poor, so he is sad.

3. His parents are rich, so he is happy.
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BelieverSometimes I have difficulty clearly seeing the causal realtionship as to decide properly on the issue of whether to put a comma before the word 'so' or not. Help. I think the 1st sentence shows a clear causal relationship but not the second and the third sentences. Do you have some tips I can follow to develop an eye to see clearly?

1. He saved money so he can pay for his college education.

2. His parents are poor, so he is sad.

3. His parents are rich, so he is happy.

In these cases, the clue for the need for a comma is the change of subject. "I am rich and am going to pay for it" vs. "I am poor, and he is going to pay for it".

Others will have more to say, I am sure. I'm better at examples than I am at rules.
In addition, I think that, whenever you can replace "so" with "in order to", then no comma.

If you can replace it with "that's why", then a comma.
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Pieanne
In addition, I think that, whenever you can replace "so" with "in order to", then no comma.

If you can replace it with "that's why", then a comma.

That's where I was headed; I just hadn't thought "far enough". Thanks for the completion, Pieanne!
Don't even mention it, Philip! Emotion: smile
Another way to look at it:

An action is done so (that) a goal will be reached.
(The action is done [with the intention of / for the purpose of] reaching the goal.)

A state of affairs exists, so this results (as the consequence of it).
(The result follows naturally; the state of affairs does not exist for the purpose of creating the result.)

In the first pattern, you can add that: so that ...
In the second, you cannot add that.

CJ
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Thank you.

Let me apply your way of thinking to the original sentences and please tell me if they make sense.

1. He saved money so he can pay for his college education.

He saved money so (that) he can pay for his college education.

2. His parents are poor, so he is sad.

His parents are poor, so (that???) he is sad. (Can we add the word "that" here? I think not because no purpose is to be found here.)

3. His parents are rich, so he is happy.

Ditto.

I think Pienne said if I replace with "that's why," then a comma and that seems to work very well for No 2 and 3, but I thought if there is a causal relationship (as the consequesnce of it, as you put it), that is a "if-because" relationship, then no comma is necessary, so according to you and Pienne, my thinking was wrong.

Sorry if my question isn't written clearly.