Question

add a comma or semicolon if needed. otherwise, submit the text without any additional punctuation. after french revolutionaries overthrew their government in the late 1700s they replaced the gregorian calendar with a new republican calendar with months named for natural elements.

Explanation:

When a subordinate clause (here, "After French revolutionaries overthrew their government in the late 1700s") comes before the main clause ("they replaced the Gregorian calendar..."), a comma is used to separate them.

Answer:

After French revolutionaries overthrew their government in the late 1700s, they replaced the Gregorian calendar with a new republican calendar with months named for natural elements.