While ‘I was wondering…’ is a polite expression, I would NOT use it in a formal letter.
- I was wondering if you could let me know by Friday.
The above sentence might leave the reader wondering whether you really know what you’re talking about. It makes you come across as an insecure person. So, it may be better to be a bit more direct, while remaining polite:
- Could you let me know by Friday, please?
- I would appreciate it if you could let me know by Friday.
