In a code language, if you are there' is written as 'ter der jer', 'we stay here' is written as 'yer mer ner', 'we are late' is written as 'ser ner der', and I stay there' is written as "yer fer jer', then how would you stay late' be written in this language?

ter yer ser

Given; In a code language, if you are there' is written as 'ter der jer', 'we stay here' is written as 'yer mer ner', 'we are late' is written as 'ser ner der', and I stay there' is written as "yer fer jer'.

So,

YOU ARE THERE =  TER DER JER

WE STAY HERE  =  YER MER NER

WE ARE LATE  =  SER NER DER

I STAY THERE  =  YER FER JER

Thus,

Words        Code

YOU          TER

ARE           DER

THERE        JER

WE             NER

STAY           YER

HERE          MER

LATE           SER

I               FER

So, the code of YOU STAY LATE is TER YER SER