Question

1. One of the most common ways to agree in English is to say "Yes" then go on to add another reason why you agree with "and…"

2. One of the most common ways to agree in English is to say "Yes," then go on to add another reason why you agree with "and…"

3. One of the most common ways to agree in English is to say "Yes", then go on to add another reason why you agree with "and…"
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This is about how to use a punctuation. Which one is gramatical? Please notice the punctuation before 'then.'

Answer 1

You'd use the punctuation in 2 in American English.

You'd use the punctuation in 3 in British English.

Read the bold sentence and then the paragraph immediately beneath it here:
http://grammar.ccc.commnet.edu/grammar/marks/quotation.htm