Often people think of verbs as ‘doing’ words. This is true for the majority of verbs but not all verbs. it is more accurate to think of verbs as words that have different tenses (past, present, future), e.g.
Is is not a ‘doing’ word. However, it has a past tense (was) and future tense (will be).
This web-site gives a good explanation on what parenthesis is and how to use it.