Why did they use で here?


I was listened to [Bump of chicken's song,
Parade](https://youtu.be/bAo-1Wsx4PY). I tried to figure out the grammar they
used on each sentence and this phrase catch my attention.

忘れないで 忘れないで 心だけが世界。

Why do they use で here ? Is it work as " です” or て form or what?




> 「Verb in [未然形]{みぜんけい} (Imperfective Form) + ないで」

expresses a soft-spoken prohibition. " ** _Please don't forget_**."

「[忘]{わす}れ」 is the 未然形 of the verb 「忘れる」. The 未然形 is the form to which 「ない」 can
be attached directly.

