Not necessarily: chưa hẳn, không hẳn

Chưa hẳn and không hẳn mean "not necessarily, not quite". They push back gently on a conclusion without claiming the opposite. They go right before the part you doubt, often with là: Mắc chưa hẳn là ngon, expensive doesn't necessarily mean good.

On its own, Không hẳn is a handy reply, "Not exactly", followed by what's really true: Không hẳn, anh chỉ hơi mệt thôi.

The two are close. Không hẳn says it simply isn't fully so. Chưa hẳn has chưa (not yet) inside, so it leaves room: maybe, but you can't take it for granted. It often pairs with đã: Học giỏi chưa hẳn đã làm giỏi, being good at school doesn't automatically make you good at the job.

Hẳn alone means "surely": hẳn là is "it must be". The negative turns that certainty into doubt.

The everyday neighbour is chưa chắc, "it's not certain", very common in Saigon: Chưa chắc ảnh tới đâu, he might not come. Chưa chắc is about whether something will happen; chưa hẳn is about whether a judgment holds. You'll also see chưa hẳn in opinion pieces, so it's at home in careful writing as well as in speech.