could you
couldja
/ˈkʊdʒə/AssimilationReduction
In real speech
What happens to the sounds
d + y blend into /dʒ/ (“j”), and you weakens to /ə/: could you → “COULD-ja”.
When to use it
Polite requests: Couldja pass the salt? Even formal speakers sound like this when talking fast. Written could you.
Casual → formal
Slang
couldja
Neutral
could you
Formal
would you mind
Sound notes
Assimilation: neighbouring sounds change each other, e.g. t + y → “ch”, d + y → “j”.