YOU SAID:

I sit in a class for a entire day

INTO JAPANESE

私は一日のクラスに座っ

BACK INTO ENGLISH

I sat in the one-day class

INTO JAPANESE

私は1日のクラスに座っていました

BACK INTO ENGLISH

I was sitting in one day class

INTO JAPANESE

私は1日のクラスに座っていました

BACK INTO ENGLISH

I was sitting in one day class