Could I help you with something?
Did you need help with something?
Are both okay and polite?
Yes and yes.
Why the 2nd sentence in the Past Tense?
Please, show me every single one of my mistakes. Thank you.
Originally Posted by NortT
Why is the second sentence in the past tense?
It means the same as 'Do you need help...?' but is seen by many as more polite.