는 and 은 are topic markers whereas 가 and 이 are subject markers. Look up talk to me in korean video on youtube they explain the difference very well.
I looked it up in google translate. 자는 translated to I am. 저가 translated to self.
Don't mind so much about the subject, in Korean you can skip it Apart i saw a video of a native, and she say that sometimes they don't use the particles
Literal translation. Swimming. I am swimming cause i am still doing it in this moment.