"I was swimming today."
Translation:आज मैं तैर रहा था ।
Why is the time the subject in most these sentences ie aaj, parason (day before yesterday) i would've thought mai aaj tair rahee thi
मैं is still the subject, this is just another way of saying the sentence. Word ordering in sentences isn't always a rigid rule, you'll hear things being moved around a bit from time to time.