When translating "I was busy yesterday," why is this incorrect?
昨日が忙しかったです
Isn't it the same as
私は昨日が忙しかったです
When translating "I was busy yesterday," why is this incorrect?
昨日が忙しかったです
Isn't it the same as
私は昨日が忙しかったです
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?'
In English, we sometimes say things like "yesterday was busy", but if you think about it, we're not actually talking about the day itself. We don't mean that yesterday was a particularly different kind of day than other days were for everybody, in general. What we actually mean is "I was busy yesterday". That is, we're talking about our own state, so really "I" should logically be the subject of the sentence, not "yesterday".
This is why「昨日が忙しかったです」is wrong in Japanese. It actually says (literally) "Yesterday was busy". That is, "yesterday" had a lot of things it needed to take care of, or "yesterday" didn't get a lot of time to itself, etc.
What you actually want to say is "yesterday, I was busy", or "speaking about yesterday, I was busy". If we were to spell that out completely in Japanese, that would be said as「昨日、私が忙しかったです」or「昨日は私が忙しかったです」(note that 私 is always the subject (が)). However, things like 私が are usually left out if they're fairly obvious (which they are here), so this would usually just be said as「昨日、忙しかったです」or「昨日は忙しかったです」instead. You can see, though, that at no point would 昨日 actually logically be the subject of anything in this sentence.