Learn Farsi: وای، ببخشید!

It comes from the verb بخشیدن (bakhshidan), meaning "to forgive" or "to pardon." Literally, ببخشید is a polite command meaning "Forgive [me]!", using the formal "you" (شما) ending.

Why is there a ب (be-) at the beginning of ببخشید?

The ب (be-) prefix marks the imperative (command) form in Farsi. It is attached to the present stem بخش (bakhsh), followed by the plural/formal ending ید (-id).

What kind of emotion does وای express here?

وای (vay) is a versatile exclamation used for sudden surprise, realization, or dismay (like "Oh!", "Yikes!", or "Oh no!"). In this sentence, it expresses the sudden realization that you've made a minor mistake or perhaps bumped into someone.

Does the colloquial Tehrani accent rule (changing the "ā" sound to "u") apply to the vowel in وای?

No, it doesn't. The colloquial shift from "ā" to "u" only happens when the "ā" vowel is immediately followed by a nasal consonant—specifically م (m) or ن (n). Because the vowel in وای is followed by a "y" sound, its pronunciation does not change to "u".