They all mean "otherwise", and beyond that 要不 is more colloquially used I'm not sure how they differ in usage or meaning.

  • The only thing I can add is that 否则 is bit more formal/literary than the others.
    – imrek
    Jun 24 '15 at 11:47
  • 2
    "实用汉语近义虚词词典"不然(连)/否则(连)/要不(连)[不同]I。"不然、否则"可以跟"的话"合用,"要不"不能:1。快让张阳上场,不然的话,我们就输了。(否则✓要不✕)2。不能再乱花钱了,不然的话,这个月的钱肯定不够用了。(否则✓要不✕)3。别闹了!否则的话,我就不客气了。(不然✓要不✕)II。"不然、否则"后面可以跟"就是。。。"的判断句。"要不"不能:1。我们应该好好学习,不然就是浪费时间和金钱。(否则✓要不✕)2。你一定要收下这个礼物,否则就是看不起我。(不然✓要不✕)III。"不然、要不"还可以引出别一种选择或可能性,表示一种建议或判断,相当于"要不然";"否则"没有这种用法:1。他们可能出去吃饭了,不然就是去散步了。(要不✓否则✕)2。你可以坐地铁去,不然坐公共汽车去也行。(要不✓否则✕)3。今天是周末,阵兰可能去打排球了,要不就是爬山了。(不然✓否则✕)3。你去找张阳吧,要不去找王飞,他们俩都会修电脑。(不然✓否则✕)iciba has these examples for 不然 not so ; not the case。Actually this is not so.其实不然。No, it's not as simple as that.不然, 事情没有那样简单。(not interchangeable with the others)
    – user6065
    Jun 24 '15 at 13:14
  • 1
    @S.Rhee You should make that an answer.
    – NS.X.
    Jun 24 '15 at 17:04
  • comments are easier to write, 写评论比写回复要容易得多,
    – user6065
    Jun 25 '15 at 5:06

否则 is often time more formal and comes across as more strongly worded (directly admonishing the listener). Much like saying "you need to do X, otherwise/if not, Y will happen (as a consequence)."

E.g., 今天你必须来参加会议,否则后果自负

要不 comes across as the softest (has the same as either/or, in the same manner as a suggestion), but it might the nuance of sounding as if one is being admonished, depending on the context. Much like saying "you (either) need to do X, or Y might happen."

E.g., 今天你必须来参加会议,要不老板会生气的

However, note that 要不 is (more commonly) used to as a way to directly or indirectly ask the listener for consent (based on the context it's used). Seeing the 吧 typically will indicate that this is a suggestion.

E.g., 一个人看电影没意思,要不我们俩一起去看吧.

不然 is often used when you want to assert a matter of fact (typically in a neutral manner), the manner of speaking is probably somewhere between 否则 and 要不 in terms of strength. Much like saying "you need to do X otherwise Y might happen."

E.g., 今天你必须来参加会议,不然老板会生气的

要不然 is a about the same as 要不, but in a bit stronger tone than 要不. Much like saying "you need to do X, or otherwise/or else Y might happen." In my opinion, 要不然 can be used interchangeably with 要不, but only in when used in the context of "otherwise."

E.g., 今天你必须来参加会议,要不然老板会生气的


Let me clarify it a bit:

  1. 否则. 否=not(negative), 则=then. 否则=if not then.

  2. 要不然(要不 and 不然 are simplified forms of it). 要不=要是不=if not. 然=like this/that.

So there are almost no difference. You can switch one by another in modern Chinese. 要不然 is weaker than 否则 in general.

  • Why was this downvoted? It was a VERY informative/illuminating answer. Dec 8 '21 at 22:38

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