When translating 'I was busy yesterday' Why is 昨日が忙しかったです Incorrect? Isn't the same as
Because the subject is changed from 'I' to 'Yesterday'. Please translate it into 「私は昨日忙しかったです」. Omitting the subject,「昨日は忙しかったです」is more natural in daily conversations. But 「昨日が忙しかったです」 is an expression to identify the day you were busy. So it is correct for the answer to the question 'Which day was a busy day for you?'