I understand the meaning of the phrase "keep your seat belt fastened while seated" which you can see in the aeroplane written on the front seat.
But I absolutely can't figure out the grammar of the phrase, of the part "while seated".
Can anyone explain why this form of verb is used? Doesn't it sound strange to a native speaker?
As far as I understand there are several things in this phrase that confuse me:
- Subject : If the subject is missed in the last verb (to seat), it should be the same subject as in case of the first verb (to keep) or of the second verb (to fasten). But to seat is a causative verb, so it's not true.
- Voice : It seems to be active, but it should be passive.
Thanks in advance!