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.'

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

Out of the three options provided, option 2 is the correct grammatical choice.

The correct way to punctuate the sentence is to use a comma (,) before the word "then". Here's the corrected version of option 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…'"

Using a comma before "then" is the standard punctuation rule in English, as it helps to separate the introductory clause ("One of the most common ways to agree in English is to say 'Yes'") from the following independent clause ("go on to add another reason why you agree with 'and…'").

Using a comma before "then" in this sentence ensures clarity and proper structure.