并非(bìng fēi) - by no means, is not at all in Mandarin

并非 (bìng fēi) is an emphatic written 不是. It does not merely say that something is untrue, it pushes back against an assumption the reader is likely to be carrying: you thought X, and in fact X is not the case.

That contradicting force is why it turns up in essays, reports and news copy, usually followed by the thing being denied and often by a correction: 并非如此，而是... (not so, but rather). It also negates whole clauses comfortably, including other negatives, as in 他并非不想来, which is not a double negative to untangle so much as a polite way of saying the reason lies elsewhere.

Two traps. First, the second character is 非, not 不, so the negation is already inside the word and you never add another 不 after it. Second, keep it out of speech: at a dinner table 不是 or 其实不是 is what people actually say, and 并非 lands like someone reading aloud from a leader column.