Final answer:
The correct verbs for the sentence are 'sets' and 'cast', making the complete sentence: 'When the sun sets, the trees cast long shadows'.
Step-by-step explanation:
The correct verbs to complete the sentence "When the sun (set, sets), the trees (cast, casts) long shadows" are 'sets' and 'cast'. The sentence should read: When the sun sets, the trees cast long shadows. The verb 'sets' agrees with the singular subject 'the sun', and 'cast' is used with the plural subject 'the trees'.
The verbs 'set' and 'cast' respectively indicate present tense and the base form which is fitting for the subject-verb agreement in this sentence.