"I am surprised": passive voice or adjective?

The sentence is: "I am surprised." I wonder why it cannot be considered as the passive form of "Someone surprises me." If it is true that it is in passive form, then why do people say that surprised is an adjective in that sentence? Please clarify this. I'm in confusion.

Answer

Well, of course it can be considered as a passive voice construction.  The active voice equivalent is quite easy to find:

• I am surprised.


• Something (or someone) surprises me.

The sentence can be understood in this manner, but that doesn't mean that it can only be understood this way.  There's another possibility:

• I am surprised.



• I am a surprised man.

The word "surprised" is a participle.  Participles and participial phrases can modify nouns in much the same way as adjectives.  Some grammar books simply call them adjectives when they're used this way. 

If I am a happy man, I can simply say "I am happy."  If I do say that, then "happy" can be understood as a predicate adjective subject complement. 

We can understand the "surprised" of your original sentence in the same way. 

As it happens, "surprised" is a stative verb -- or, at least, it's a verb that's often used in a stative sense.  Both the passive voice interpretation and the subject complement interpretation are available for your original sentence.  For a stative verb, the subject complement interpretation is likely to be more useful and, for many, the more obvious interpretation.
 

it's not a matter of which interpretation is correct.  Both are correct.  It's only a matter of which interpretation makes more sense in context.  If you can see both interpretations easily, then you should be able to easily choose between them as context requires.  You may also find that, in many contexts, the overall meaning of the entire passage won't change no matter which interpretation you choose.
 

You may also want to note that "to be" isn't the only possible copular verb.  There's a handful of verbs that work in copular constructions:

• He is surprised.


• He seems surprised.


• He looks surprised.


• He sounds surprised.

You're free to interpret "He is surprised" as a passive voice construction.  For every other verb that fits this same pattern (and my examples are far from exhaustive) only the subject complement interpretation is obvious.