what is the difference between "in" and "after" (two day)
in two days I have an exam
AND
after two days I have an exam

By the way, will it sound natural if I say time first then the rest words like this?

Question

what is the difference between "in" and "after" (two day)
in two days I have an exam
AND 
after two days I have an exam

By the way,  will it sound natural if I say time first then the rest words like this?

Answer

In two days, I have an exam. 
I have an exam in two days.

Use one of those, not after, to describe this.

Answer

IN 2 days implies the act will happen on the 2nd day.
AFTER 2 days means 2 days must pass first, then the act can take place.

In 2 days it will be my birthday.
After 2 days I can take the test again.