Are these sentences correct? which one is correct?
1. I have studied english yesterday but I forgot what i studied.
2. I studied english yesterday but I forgot what i studied.
3. I was studying english yesterday but i forgot what i studied.
4. I had studied english yesterday but i forgot what i studied.
5. I had been studying english yesterday but i forgot what i studied.
6. I have been studying english yesterday but i forgot what i studied.
Help me to chose or make a good sentence.