Andy and I are bestfriends since gradeschool.--> I believe this is correct.
Andy and I is upset.
Janice and Shiela is fighting almost everyday.
Janice and Shiela are fighting almost everyday.
I heard her sang once.
I heard her sing once.
For me, she's the best teacher.
To me, she's the best teacher.
Please explain why some of the sentences are wrong. Thanks! :)