I wrote the following sentences (in an exercise in which I had to tell how I spent the day):
ごごに、にほんごをべんきょうしました。
ばんに、ばけもののこをみました。
A Japanese person corrected them like this:
ごごは、にほんごのべんきょうをしました。
ばんには、ばけもののこをみました。
As I was speaking about me in the previous sentences, I thought the subject (me) was implied. I don't understand why the subject here is the afternoon itself or the night?