Final answer:
In the sentence provided, 'should go' is the simple predicate, conveying the action pertaining to the subject 'they'.
Step-by-step explanation:
The simple predicate in the sentence "They should go to the movies after studying" is should go. A predicate is the part of a sentence that tells us what the subject is doing or what is being said about the subject. In this case, the action or state of being conveyed is 'should go,' expressing what 'they' ought to do. Predicates can help us clarify statements and understanding them is essential for determining the structure of a sentence and for proper sentence construction.