Why does 人 have a different tone when it's part of the word 大人?
Google Translate: 人 / rén/ ; 大人 /dàrén/. I don't see the different pinyin for 人


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  CantoDict :   
  
  人/ren2/ 
  
  大人 /da4 ren5/2/
  
  (1) adult; grown-up
  
  (2) Your Honour; Your Excellency
  
  (3) [literary] Dear (Father/Mother)
  
  (4) great man; person of high rank or noble mind
  
  Note:
  
  1: Pinyin da4 ren5 is used for first two meanings; da4 ren2 for latter two.
  
  2: "Your Honour" is mainly used on a judge.


In Mandarin:


人 on its own is pronounced as /rén/
When 大人 meant "adult" or "your Honour" 人 is pronounced as /ren/.
When 大人 meant "Dear" or "great man" 人 pronounced as /rén/


So, you only need to remember 人 pronounces differently as /ren/ only when you mean to say 大人(adult) or 大人 (your Honour). In all other instances 人 is pronounced as /rén/