He returned the mobile that he stole.

[the batman learner]

He returned the mobile that he stole.
He returned the mobile that he'd stolen.

What's the difference and are these both correct and mean the same?

 

[emsr2d2]

We have asked you several times to stop just posting random sentences in which the only difference is past simple v past perfect. You haven't provided any context or told us in what real life situation you found yourself when you needed to use this sentence.

 

[Tarheel]

He returned the mobile that he stole.
He returned the mobile that he'd stolen.

What's the difference? Are these both correct, and do they mean the same thing?

Absent context, they mean the same thing. You can view the last part as a noun phrase. Thus "the mobile that he stole" and "the mobile that he'd stolen" are the same thing. (You don't need "that" in the second sentence.)

 

[Barque]

He returned the mobile that he stole.
He returned the mobile that he'd stolen.

What's the difference and are these both correct and mean the same?

Sometimes there can be a difference. Sometimes there's none. It depends on context. There's no rule or "tip" on when to use which. You have to learn it by experience.