Some verbs may function both as linking verbs, taking a subjective complement, and as transitive verbs, taking a direct object. Note that the meaning of the verb usually changes with the change in function.
Here are some tips to help you figure out whether you're dealing with a subjective complement or a direct object. Consider the part of speech of the constituent that follows the verb. If it's an adjective--not a noun phrase--it can't be a direct object:
Alfred proved loyal.
If what follows the verb is a NP, consider the relationship between it and the subject NP. If the subject NP and the NP following the verb are coreferential (refer to the same thing), then what you've got is a subjective complement:
Alfred proved a loyal friend.
Note that the statement "Alfred is a loyal friend." is true.
If the NP following the verb is not coreferential, then it is a direct object.
Alfred proved Euclid's theorem.
Note that the statement "Alfred is Euclid's theorem." is false.
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