Inverse marking
Mirja has a crosslinguistically very unusual argument role marking system. In many languages, it's assumed that whatever is the subject is going to be the topic, and often strategies such as reordering have to be used to mark situations when the subject is not in fact the topic. Mirja does the inverse of this—it assumes that whatever is marked as the topic is going to be the subject, and requires you to mark the verb if that assumption does not hold. In effect, topic marking is allowed to do double duty as subject marking—but when the verb is marked with the inverse marker -e, that signals that the topic marking is no longer performing that double duty and the actual subject is somewhere else.
There are several reasons why the subject and topic may fail to coincide. It's possible that some other argument is the topic:
su
3sg[TOP]
no-rV
1sg-OBL
karu-t-e
be.visible-PAST-INV
'I saw him'
It's possible that there is no topic at all:
halli-rV-t
who-OBL-FOC
va-t-e-wwe
do.it-PAST-INV-Q
'Who is it that did it?'