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There are two kinds of linking verbs: those that express a state and those that express a result.
Express a state
am, is, are, was, were, be, being, been (any any combination that ends with be or been: has been, have been, had been, will be, shall be, may be, would have been, should have been, would be). Express a result: These are called "resultative"
act, appear, be, feel, lie, look, remain, seem, smell, sound, stay, taste, become, get, grow, fall, prove, run, turn
A resultative verb can function as either a linking verb or an action verb. Here's a test you can use to help you decide: Replace the resultative verb with am, is, are, was, etc. For example, let's try the verbs 'grow' and 'look':
The man grows flowers.
=> The man is flowers. (Not OK)
('grows' is not a linking verb here. It's an action verb)
The man grows tired.
=> The man is tired. (OK)
('grows' is a linking verb here.)
She is looking at the flowers.
=> She is at the flowers. (Not OK)
('is looking' is not a linking verb here. It's an action verb)
She is looking good.
=> She is good (i.e., looking). (OK)
('is looking' functions as a linking verb here.)
Here's a test to help you decide whether the word that come after the linking verb is a noun or an adjective: Replace the linking verb with the verb seem. Only adjectives fit grammatically in the position:
He is a doctor.
=> He seems a doctor. (Not OK)
('a doctor' is not an adjective)
He is happy.
=> He seems happy. (OK)
('happy' is an adjective)
tdol is correct. :-D 'rough' is an adverb in that context. 'rough' describes how he plays, not what kind of person he is. Here are two test that might help you to determine if the word is an adjective or an adverb:
Adjective Test: Ask the question, What kind of?
EX: He is a rough football player.
Q: What kind of football player is he?
A: He is a rough one. (Adjective)
Adverb Test: Ask one of the questions, How, When, Where, Why?
EX: He plays rough.
Q: How does he play?
A: He plays rough. (Adverb)