The main character (usually a woman)
Because of some accidents, suddenly traveled to the ancient times
Can be the history of the times
Can also be the imaginary times
Then it happens a story (usually based on the love)
Traveling through the time can be with
There is also the soul travel (soul into the body of the people of that era)
The second case there are a lot of things can be changed
such as the age, gender... all of them can be different from the original protagonist. All of them can be different from the original protagonist
But the only thing that remains unchanged is the original protagonist's thoughts
That is, the protagonist can look at the problems of other eras from the perspective of a modern person
(Usually, the era that the protagonist crosses over to is more backward than the modern one)