The pinyin for 人 is rén but when it's part of 大人, it's written as dà ren. This is the only word with 人 where that happens (at least in the list of words I went through). Why is there a change in tone?
Google Translate: 人 / rén/ ; 大人 /dàrén/. I don't see the different pinyin for 人
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
1: Pinyin da4 ren5 is used for first two meanings; da4 ren2 for latter two.
2: "Your Honour" is mainly used on a judge.
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/