Answer:

Subject-Verb Agreement:
In the given sentence, the subject "Tina" is followed by the verb "weed." However, the verb tense is not suitable for the sentence as it uses the base form of the verb instead of the future tense.

Future Tense:
To indicate an action that will happen in the future, we need to use the future tense. In English grammar, the future tense is formed using the auxiliary verb "will" followed by the base form of the verb. Therefore, the correct verb form to use in this sentence is "will be weeding."

Explanation:
- In the given sentence, "Tina" is the subject and "weed" is the verb.
- Since the action of weeding is scheduled to take place in the future, the base form of the verb "weed" needs to be changed to the future tense.
- The correct future tense form of the verb "weed" is "will be weeding."
- The auxiliary verb "will" indicates the future tense and is followed by the base form of the verb "be."
- The present participle form of the verb "weed" is "weeding," which is used after the auxiliary verb "be" to form the correct future tense.
- Therefore, the correct sentence is "Tina will be weeding in the garden at 5.30 p.m. tomorrow evening."

Summary:
To maintain subject-verb agreement and indicate an action that will happen in the future, the correct verb form in the given sentence is "will be weeding." This form uses the future tense, with "will" as the auxiliary verb and "be weeding" as the verb phrase.