并非【bìngfēi】

并非 means "is not", with the edge of correcting what someone might assume. 非 is the written "is not", so 并非 stands where speech would say 并不是. It belongs to essays, reports and formal statements.

并非…而是

The most frequent frame rejects one account and puts another in its place.

Not all

并非所有…都 and 并非都 deny "all" rather than each item: some are, some aren't.

并非不

A second negative makes a guarded "yes", often followed by 而是 or 只是 with the real reason.

The colloquial counterpart, 并不 and 并没有, works the same way in speech.

Judgements, not events

并非 denies what something is, not whether an action took place. For an event that didn't happen, use 并没有.

• × 他昨天并非去 → 他昨天并没有去

Since 非 already means "is not", 并非是 doubles the verb. You will see it in print, but careful writing leaves out the 是.