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.