不一定是的英文

not sure~

not sure not certainlynot alwaysincertitude indeterminacy indetermination uncertainty unsureness

It depends.

That depends...

It depends. 这句直接翻译是“不一定”、“看情况”，可能更接近你想要的用法。

应该是not really国外都这样说的

not necessarily

Not necessarily 绝对标准大学英语教材中可见

uncertaintymaybe not benot necessarily例：Every rascal is not a thief,but a thief be a rascal.每个无赖不一定是盗贼;每个盗贼必定是无赖。Let's eat now and maybe we could catch a movie later. 咱们现在就吃,说不定吃完能赶场电影.Clouds do not necessarily forecast rain.云层未必预示下雨。