A question about commas

I just thought I'd share this recent email exchange:

Student question

I have a question about "if" and commas.

What is the differet between this two sentences.

1 I would get married, if I were you.

2 I would get married if I were you.

My response:

The comma does make a slight difference. A comma tells a reader to pause, and it separates out parts of a sentence.

Sentence number 1 encourages the reader to stress IF. Stressing IF makes what follows seem less likely, or in this case, emphasises the fact that "I am not you".

There are other occasions when commas will make a much bigger difference to meaning. Can you see a difference between these two sentences:

1. I like student who email me questions about grammar.

2. I like students, who email me questions about grammar.