Translation:I thought that my mother was in the car.
Irish is V/S/O, where the object of a transitive verb (like think/thought) can be another clause.
Verb (what happened): Thought - Cheap Subject (who made it happen): I - mé Object (the thing that happened): my mother was in the car.
So the object is another sentence…
Verb: was - bhí, or go raibh if it’s the object. Same reason it’s go bhfuil instead of tá in present tenses. Subject: my mother - mo mháthair Object: It’s a location this time, “in the car” - sa charr.
Cheap mé go raibh mo mháthair sa charr.
When you ask about raibh, are you mistaking it for “riamh,” thinking they mean “ever?”