What is the past participle of is?

The verb "to be" does not have a past participle in the same way that regular verbs do. The past participle of "to be" is been.

Here's why:

* Regular verbs form their past participle by adding "-ed" (e.g., walk - walked).

* Irregular verbs like "to be" have different forms for the past tense and past participle.

Example:

* "He has been to the store." (past participle)

* "He was at the store." (past tense)