If I knew we were going to have a test today, I would have studied harder yesterday.
If I had known we were going to have a test today, I would have studied more yesterday.
どちらも「今日テストがあると知っていたら、昨日もっと勉強したのに」という意味です。
「今日テストがある」の部分は
we were going to have a test today
のように表現します。
ーIf I knew we were going to have a test today, I would have studied harder yesterday.
＊こちらは口語的な言い方です。会話ではこのように言うネイティブが多いと思います。
ーIf I had known we were going to have a test today, I would have studied more yesterday.
＊こちらは文法的に正しい言い方です。
ご参考まで！
- If I had known there was a test today, I would have studied more yesterday.
- *If I had known there was a test today, I would have studied more yesterday.*
（今日テストがあると知っていたら、昨日もっと勉強したのに）
- *If she had told me about the meeting, I wouldn’t have been late.*
（彼女がその会議について教えてくれていたら、遅刻しなかったのに）
- *If we had booked earlier, we could have gotten better seats.*
（もっと早く予約していたら、もっと良い席が取れたのに）
カナダ 
アメリカ合衆国 
