if you have to memorize a lot of information in a short time,you will soon forget the knowledge you gain.
このラストの"you gain"の部分は、"you will have gained"の方が良いでしょうか？
If you have to memorize a lot of information in a short period of time, you will probably forget most of it right away.
If you have to ... と言っているので、knowledge you gain と言うと良いですよ。
ーIf you have to memorize a lot of information in a short time, you will soon forget the knowledge you gain.
上の文はちょっとかたいので、私なら次のように言います。
ーIf you have to memorize a lot of information in a short period of time, you will probably forget most of it right away.
「短期間でたくさんの情報を覚えなければいけない場合、多分そのほとんどをすぐに忘れるだろう」
ご参考まで！
カナダ 
