Correct

必然 vs 必定

/ bìrán vs bìdìng /
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Definition

必然 as an Adjective, 必定 as an Adverb, both 必然 and 必定 mean "definite", but 必然 stresses certainty in argument, while 必定 stresses "the certainty in one's subjective mind".

Example

必然 and 必定 have different syntactical functions. 必然 can be used as different sentence elements, while 必定 can only function as an adverbial. The two words are interchangeable sometimes.
The sentences where the two words are interchangeable are limited to more formal sentences or sentences concerning abstract things.
1
Chuàng
创业
rán
必然
  
(
dìng
必定
  
)
yào
mào
fēngxiǎn
风险
  

Launching a career is very likely to take risks.

2
Shèng
胜利
rán
必然
  
(
dìng
必定
  
)
shǔ
属于
zhì
意志
jiānqiáng
坚强
de
rén
  

Success definitely belongs to those with strong will.

必然 can function as the subject or object of a sentence (indicating the objective law independent of man's will).
3
Xīn
shì
事物
dài
代替
jiù
shì
事物
  
,
zhè
shì
shèhuì
社会
zhǎn
发展
de
rán
必然
  

It is an objective law of social development that new things will take the place of the old ones.

4
guān
客观
de
rán
必然
  
,
shì
rén
de
zhì
意志
wéi
zhuǎn
转移
de
  

The objective certainty is independent of man's will.

必然 can also function as attributive and predicated with 是……的.
5
Rén
yào
shuāilǎo
衰老
  
wáng
死亡
  
,
zhè
shì
rán
必然
de
guī
规律
  

It is natural that man will get old and die. (attributive)

6
jìn
进步
jiù
huì
luòhòu
落后
  
,
zhè
shì
rán
必然
de
  

It is certain that if you do not make progress,you will fall behind. (predicate)

必定 indicates judgment or resolute will. 必然 cannot be used here.
7
Tiānbiān
天边
yún
乌云
Gǔngǔn
滚滚
  
,
dāi
huìr  
会儿
dìng
必定
huì
xià
下雨
  

The dark clouds billows across the sky,and surely it is going to rain soon.

8
nián
nèi
以内
  
,
dìng
必定
lái
  

I will marry you within a year.