How to use "will then do" and "will then"? Is "will then" means there is no choice left? Can list me some examples?
Not sure if this is right:
When the examination dates are near, the students will then do their revisions(singular).
When the chicken burgers have are finished (or "have all gone"), the students will then buy the sandwiches.
With the amendments shown, your sentences are OK. The use of "will then" suggests a direct correlation or connection between the two events. The first one suggests that none of the students will do any revision until the exam date gets close. The second suggests that none of the students will buy sandwiches until all the chicken burgers have been bought/eaten. If that is what you're trying to suggest, then you're OK. They're not perhaps the most natural English. I would have said:
The students will only start to revise once the exams are getting close.
Once the chicken burgers have all gone, only then will the students buy the sandwiches.