the はひとつしか存在しないものにつく。
a は複数存在するもののうちのひとつにつく。
This is the train which goes to London.
This is a table which was made by my father.
Ayaka is a girl who is loved by everyone.
I know the girl who came here yesterday.
I'm looking for a singer who can sing well.
This is a photo which I took in Kyoto.
This is the photo which I took in Kyoto.
a と the は明確に違う。