Final answer:
The verb in the perfect tense in the given sentence is 'will have had'. It indicates an action that will be completed by a specific time in the future, which is 'so far this spring'. Other verbs in the sentence do not denote completed actions in the same way.
Step-by-step explanation:
The verb that is in the perfect tense in the sentence 'If the rain keeps up at this rate, we will have had 30 inches so far this spring' is 'will have had'. The perfect tense is a verb form that indicates that an action has been completed before a specific time. Other verbs in the given sentence such as 'keeps', 'will continue', and 'are estimating' do not denote an action that has been completed in the past relative to a certain time in the future as 'will have had' does.
In English grammar, perfect tenses are used to show that an action or event has been completed prior to another action or event or before a specific point in time. Therefore, the verb 'will have had' reflects an action (having had 30 inches of rain) that will be completed by the time of reference (this spring).
Understanding verb tenses is critical for maintaining consistency and clarity in writing. The perfect tense, along with other tenses like present, past, and future, allows writers to accurately convey when an action or event takes place. As such, practicing with different verb forms, like those that have old past tense forms ending with 't', helps in refining one's grasp of proper tense usage.