Student or Learner
Here is a sentence:
Please rest assured that we would have informed you at the time of your purchase, if we had known about this then.
1. Is this grammatically correct?
2.What does "rest assured" mean?
P.S.: If the sentence sounds unnatural to you, please let me know.
***Not a teacher***
I would say this is grammatically correct.
'Rest assured that' - means 'you can be totally confident that' and carries with it a sense of being so confident that you do not have to worry further about it - be at total peace about it.
I think the sentence sounds ok. If I was writing an official letter and wanted to be pedantic, I would replace 'then' with 'at the time', because ending the sentence with 'this then' sounds a bit clunky to me.
Please rest assured that we would have informed you at the time of your purchase, if we had known about this at the time.
The only thing that doesn't sit right with me is that it seems there is still a problem, so I can't rest assured until the problem has been fixed. It's not enough for me to know that you would have told me if you had known yourself; I need it fixed.
If you want to assure me that you would have told me if you'd known, then you can use "Please be assured." However, I won't relax (and "rest assure") until it's fixed.
A better use of "rest assured" is "Rest assured, the problem has been entirely resolved and we've taken steps to ensure it won't happen again."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.