The word "then" is a conjunctive adverb. When it separates independent clauses, it should be preceded with a semicolon (;) and usually followed by a comma (,). With most conjunctive adverbs--such as "however" and "therefore," this is not a difficult rule to follow, but with "then" it is problematic. For example:
John went to the store; then, he bought an apple.
Because "then" and "and" are so similar sounding, most native speakers in the United States treat "then" as a regular conjunction and write the sentence as follows:
John went to the store, then he bought an apple.
It becomes even more confusing when native speakers leave the subject in the second independent clause "assumed."
John went to the store, then bought an apple.
This apparent error is so common in written material in the USA that I am not sure what is correct. I can't find an answer in grammar books. As a matter of fact in some grammar books the author quotes the rule as stated above then in practice violates it. Such books are Woe Is I and Painless Grammar.
My question is: What is the correct way to write the following sentence?
John went to the store, then bought an apple.
Could I consider this a sentence with a compound verb and leave out the comma, treating "then" as a regular adverb? "John went then bought."
John went to the store then bought an apple.
Thank you to whomever can help me with this.