Correct

否则 vs 不然

/ fǒuzé vs bùrán /
Strokes

Formula

1
否则的话
2
不然的话
3
要不的话
4
要不然的话

Definition

否则 and 不然 both are Conjunction, which means [otherwise].

Example

否则 or 不然 is used at the beginning of the second clause to indicate a hypothetical negation to the preceding clause, followed by the possible result 的话 can be added.
11) In example ③ a rhetorical question is used instead to indicate the possible result.

2) 否则 and 不然 are more literary; 要不 and 要不然 is more colloquial and so more often takes 的话 after it.
否则的话 不然的话 要不的话 要不然的话
1
Xìngkuī
幸亏
le
fángzhēn
预防针
  
,
fǒu
否则
  
(
rán
不然
  
)
  
(
yào
  
)
  
(
yàorán
要不然
  
)
dìng
一定
huì
bìng
得病
  

But for the vaccine, I would have been ill.

2
必须
yònggōng
用功
  
,
fǒu
否则
  
(
rán
不然
  
)
  
(
yào
  
)
  
(
yàorán
要不然
  
)
de
huà
  
,
chéng
成绩
kěndìng
肯定
shàng
  

You must work hard, or you will not catch up with the others.

3
Lǎoshī
老师
yīnggāi
应该
rènzhēn
认真
bèi
备课
  
,
fǒu
否则
  
(
rán
不然
  
)
  
(
yào
  
)
  
(
yàorán
要不然
  
)
zěnme
怎么
néng
jiào
hǎo
ne
  
?

A teacher must prepare his lessons well, or how can we expect him to teach well?

否则 and 不然 at the beginning of the second clause can also mean "either......or......".
In the three examples, any of the three other words can be used, and 的话 can also be added.
4
men
他们
jīngcháng
经常
zài
wǎnshang
晚上
xiántán
闲谈
  
,
fǒu
否则
  
(
rán
不然
  
)
  
(
yào
  
)
  
(
yàorán
要不然
  
)
jiù
kuàir  
块儿
tīng
yīnyuè
音乐
  

They usually either chat or listen to music together at night.

5
jiā
大家
dōu
tǎoyàn
讨厌
  
,
jiànmiàn
见面
lěnglěng
冷冷
de
diǎntóu
点头
  
,
fǒu
否则
  
(
rán
不然
  
)
  
(
yào
  
)
  
(
yàorán
要不然
  
)
jiù
zhuāng
méi
kànjiàn
看见
zǒukāi
走开
  

All of us dislike him.When we met him, we either nodded him coldly or pretended not to see him.

6
men
我们
kāi
liánhuānhuì
联欢会
  
,
fǒu
否则
  
(
rán
不然
  
)
  
(
yào
  
)
  
(
yàorán
要不然
  
)
jiù
chū
出去
wánwán
玩玩
  

Let's either have a get-together or go out for fun.